WEBENCH® Tools/UCC28742: MOSFET selection for Valley Switching

Clint Appling

Part Number: UCC28742
Other Parts Discussed in Thread: TL431, UCC28600, UCC28C42, UCC25600

Tool/software: WEBENCH® Design Tools

I am having a hard time finding a MOSFET for the UCC28742 that will allow for valley switching.
How critical is it to achieve this?
What are the side affects of not valley switching besides lower efficiency and more power dissipated by the MOSFET?

Do you know of any MOSFETs that will better achieve valley switching? So far even with $40 parts, I can not achieve this and be within the current rating of the MOSFET.

MOSFET Switch, Q
Reflected Voltage, V_reflected =	640.0	V	Variable definitions refering to the diagram on the sheet of SCHEMATIC AND BoM
Leakage Spike Voltage, V_LK =	416.0	V
Required Drain to Soure Voltage Rating , V_DSrated =	2156.0	V
Leakage Spike Voltage / Reflected Voltage, Ratio = V_lk / V_reflected	0.65		User input and initially around 0.5 to 0.7
MOSFET Rated Drain to Source Voltage, V_DS =	2500	V
Output Capacitance of Selected MOSFET, C_OSS =	77	pF		Design will not Valley Switch
Drain to Source On-Resistance of Selected MOSFET, R_DSon =	40	Ω
MOSFET Fall Time, t_f =	39	ns
MOSFET Turn Off Delay Time, t_Doff =	132	ns
MOSFET Total Gate Charge, Q_g =	65	nC

Regards,
Clint

over 6 years ago

0 Mike O' over 6 years ago

TI__Guru* 86266 points

I am not sure why you are having issues with Valley switching?

Could you provide schematic and power levels?

Could you also take waveforms of the Aux winding, Drive Pin and Current Sense pins so we can see where the device is actually switching?

Regards,

Mike

0 Clint Appling over 6 years ago in reply to Mike O'

Prodigy 245 points

I do not have waveforms to view at the moment, because I am still in the process of sorting out the correct components using the design tool.
Once I have these correctly set, I will move to layout and PCB manufacturing and test, but I want to make sure I have setup the design correctly.

Currently, I can not find a MOSFET to achieve valley switching at the current level needed.
The design takes a DC input voltage (725V to 1075V) from a large capacitor bank as the input.
The output needs to be 15Vdc at 1.3Amps. The output voltage range during operation and various load states must remain between (14.5V to 15.5Vdc) with the typical value being 15Vdc.

One thing I think I need to look at is revising the Vaux turns since I originally thought I'd just need 40 turns just like the secondary to have a 15V aux, but this calculator is showing I need more like 65 turns which is quite a bit more?

I am also looking at a startup circuit to allow the circuit to start up quickly.

UCC28742 DESIGN CALCULATIONS
The Values Entered by the User on the DESIGN INPUT Page are Used in the Design Calculations
ALL Gray CELLs from BLUE CELLS ARE USER INPUTS - changes only made to BLUE cells
INPUT
Input Voltage Type	AC or DC:	DC		User Input Values From Design Input Page
Minimum Input Voltage	V_INPUTmin =	725	VDC
Maximum Input Voltage	V_INPUTmax =	1100	VDC
Nominal Input Voltage	V_INPUTnom =	1073	VDC
Minimum Line Frequency	f_LINEmin =	0	Hz
Minimum Input Voltage for Start-Up	V_INPUTrun =	720	VDC
Minimum Peak Bulk Input Voltage	V_BULKmin =	725.0	V
Maximum Peak Bulk Input Voltage	V_BULKmax =	1100.0	V
Nominal Peak Bulk Input Voltage	V_BULKnom =	1073.0	V
Turn-On Peak Bulk Input Voltage	V_BULKstartup =	720.0	V
Line Cycle Period, maximum, (corresponding to f_LINEmin)	t_LINE =	n/a	ms
OUTPUT
Regulated Output Voltage, Constant Voltage Mode	V_{OUT_CV} =	15.0	V	User Input Values From Design Input Page
Full Load Rated Output Current	I_OUT =	1.3	A
Target Constant Current Mode Output Load Threshold	I_{OCC_target} =	1.5	A
Target Minimum Output Voltage During Constant Current Regulation	V_{OUT_CC} =	14.0	V
	V_{OUT_CC} =	14.0	V
Allowable Output Voltage Drop During Load-Step Transient in Constant Voltage Mode	V_OUTΔ =	0.05	V
	V_OUTΔ =	0.05	V
Maximum Peak to Peak Output Voltage Ripple	V_RIPPLE =	80	mV
Required Positive Load Step Transient Current	I_TRAN =	1.3	A
Maximum Allowable Response Time to Load Step Transient	t_RESP =	0.1	ms
Maximum Allowable Response Time to Load Step Transient	t_RESP =	0.1	ms
Output Over Voltage Protection	V_{OUT_OVP} =	15.5	V
Maximum Stand By Power Dissipation	P_SBtarget =	75	mW
Estimated Efficiency	η =	0.850
Output Power	P_OUT =	22.500	W
Estimated Input Power	P_IN =	26.471	W

COMPONENT PARAMETER CALCULATIONS

INPUT CAPACITOR, C_BULK
Recommended Input Bulk Capacitance	C_{BULKrecommended} =	11.25	µF
Actual Input Bulk Capacitance	C_BULKactual =	1000.0	µF	User Input
Input Capacitor Value Used in Calculations	C_BULK=	1000.0	µF
Minimum Valley Voltage on Input Bulk Capacitors	V_BULKvalley =	720.0	V
Minimum Input Capacitor Ripple Current Rating	I_CINripple =	0.1	mA
Minimum Input Capacitor Voltage Rating	V_Cin =	1100	V

INPUT FUSE
Voltage Rating	V_FUSE =	1100	VDC
Peak Input Current	I_INpeak =	0.140	A

BRIDGE RECTIFIER
Voltage Rating	V_{BRIDGE_minrating} =	1100.0	V
Current Rating	I_{BRIDGE_minrating} =	0.074	A
Forward Voltage Drop	V_{F_BRIDGE} =	1.000	V	User Input
Full Load Power Dissipation of Bridge Rectifier	P_BRIDGE =	0.1	mW

TRANSFORMER TURNS-RATIO, N_PS
Demagnetizing Duty Cycle	D_{DEMAG_CC} =	0.475		Device Parameter
Amplitude Modulation Control Ratio	K_AMnom =	4		Device Parameter
Maximum Desired Switching Frequency	f_{max_target} =	75.0	kHz	User Input
Desired Switching Period	t_{SW_target} =	13.333	µs
Resonant Frequency During DCM Dead Time	f_RES =	0.500	MHz
Time to First Resonant Valley	t_RES =	1.000	µs
Estimated Maximum Duty Cycle	D_{max_target} =	0.450
Ideal Primary to Secondary Turns Ratio	N_PSideal =	42.6316		Ideal N_PS
Actual Primary to Secondary Turns Ratio	N_PSactual =	40.000		User Input
Primary to Secondary Turns Ratio Used in Calculations	N_PS =	40.000
Actual Flyback Voltage	V_FLYBACK =	640.0	V
Allowable Leakage Inductance Voltage Spike	V_LEAKAGE =	760.0	V
Estimated Maximum On-Time	t_ONestimated =	5.804	µs
Estimated Transformer Efficiency	η_XFMR =	0.945

CURRENT SENSE RESISTOR, R_CS, PEAK PRIMARY CURRENT, I_PP
Constant Current Regulation Factor, Minimum	V_{CCR_min} =	338	mV	Device Parameter
Constant Current Regulation Factor, Nominal	V_{CCR_nom} =	363	mV	Device Parameter
Constant Current Regulation Factor, Maximum	V_{CCR_max} =	390	mV	Device Parameter
Initial estimate for L_P	L_{P_estimate} =	21732.2	µH
Recommended Current Sense Resistor Value	R_{CSrecommended} =	4.705	Ω
Actual Current Sense Resistor Used	R_CSactual =	4.700	Ω	User Input
Current Sense Resistor Value Used in Calculation	R_CS =	4.700	Ω
Power Dissipation of R_CS	P_Rcs =	17.8	mW
Maximum Current Sense Threshold Voltage, Minimum	V_{CSTmax_min} =	710	mV	Device Parameter
Maximum Current Sense Threshold Voltage, Nominal	V_{CSTmax_nom} =	770	mV	Device Parameter
Maximum Current Sense Threshold Voltage, Maximum	V_{CSTmax_max} =	830	mV	Device Parameter
Peak Primary Current, Minimum, Full Load	I_PPmin =	0.151	A
Peak Primary Current, Nominal, Full Load	I_PPnom =	0.164	A
Peak Primary Current, Maximum, Full Load	I_PPmax =	0.177	A
Actual Output Current During Constant Current Mode	I_{OCC_actual} =	1.556	A
Peak Primary Current During Light Load, FM Mode	I_{PP_FM} =	0.041	A
Worst Case Peak Primary Current	I_{PP_WC} =	0.178	A	Assumes -1%R_CS and V_{CSTmax_max}
Worst Case Peak Primary Current	I_{PP_WC} =	0.178	A	Assumes -1%R_CS and V_{CSTmax_max}
Maximum Output Current During Constant Current Mode	I_OCCmax =	1.695	A	Worst Case Estimate

TRANSFORMER PRIMARY INDUCTANCE, L_P
Calculated L_P to meet f_{max_target} with chosen R_CS	L_Pcalc =	22253.922	µH
Recommended Primary Inductance to meet t_CSLEB with chosen R_CS	L_Precommended =	22253.922	µH	Ideal L_P
Actual Primary Inductance	L_Pactual =	22000.000	µH	User Input
Primary Inductance Used in Calculations	L_P =	22000.000	µH
Actual Maximum Nominal Switching Frequency	f_max =	84.345	kHz
Actual Switching Period	t_SWactual =	11.856	µs
Actual Maximum On-Time	t_ONmax =	5.006	µs
Maximum Duty Cycle	D_MAX =	0.422
Demagnetization Time	t_DEMAG =	5.632	µs
Primary RMS Current	I_{PRI_RMS} =	0.061	A
Secondary Peak Current	I_SPmax =	6.553	A
Secondary RMS Current	I_{SEC_RMS} =	2.608	A
VDD Under Voltage Lock Out (UVLO) Voltage, Maximum	VDD_{OFF_max} =	8.150	V	Device Parameter
VDD Under Voltage Lock Out (UVLO) Voltage, Minimum	VDD_{OFF_min} =	7.350	V	Device Parameter
Recommended Auxiliary to Secondary Turns Ratio	N_{ASrecommended}=	0.610
Recommended Primary to Auxilliary Turns Ratio	N_{PArecommended} =	65.574
Actual Primary to Auxiliary Turns Ratio	N_PAactual=	40.000	User Input
Primary to Auxiliary Turns Ratio Used in Calculations	N_PA =	40.000
Nominal VDD Voltage	VDD =	15.000	V
Actual Auxiliary to Secondary Turns Ratio	N_AS =	1.000
Maximum On-Time, t_CSLEB	t_ONmax(limit)=	350.0	ns	Device Parameter
Actual Minimum On-Time (worst case for Ipp_min)	t_{ONmin(actual)}=	755.3	ns	Ok
Actual Minimum On-Time (normal for Ipp_nom)	t_{ONmin(normal)}=	819.1	ns	Normal operation ok
Minimum Demagnetizing Time	t_DEMAGmin =	1.298	µs
Minimum Output Voltage During Constant Current Mode	V_{OUT_CCmin} =	7.350	V

MOSFET, Q
Estimated leakage spike voltage	V_LK =	416.0	V	adjusted by the ratio of V_LK/V_reflected
Reflected Voltage	V_reflected =	640.0	V	Variables refer to SCHEMATIC AND BoM Sheet
Required Drain to Soure Voltage Rating , V_DSrated =	V_{DSmin_rating} =	2156.0	V	Variables refer to SCHEMATIC AND BoM Sheet
MOSFET Rated Drain to Source Voltage	V_DS =	2500.0	V	User Input Values From Design Input Page
Output Capacitance of Selected MOSFET	C_OSS =	77	pF
Drain to Source On-Resistance of Selected MOSFET	R_DSon =	40.000	Ω
MOSFET Fall Time	t_f =	39.0	ns
MOSFET Turn Off Delay Time	t_Doff =	132.0	ns
MOSFET Total Gate Charge	Q_g =	65.0	nC
Actual Resonant Frequency During DCM Dead Time	f_{RES_actual} =	0.086	MHz
Actual Estimated Time to First Resonant Valley	t_{RES_actual} =	5.783	µs
Valley Switching Achieved?	YES or NO	NO	Efficiency will be impacted
MOSFET V_DS Derating	V_DSderated =	0.862
MOSFET Continuous Current Rating	I_DRAIN =	0.669	A
MOSFET Pulsed Current Rating	I_PULSED =	1.784	A
Estimated MOSFET Conduction Losses	P_{FETconduction} =	0.151	W
Estimated MOSFET Switching Losses	P_FETswitching =	1.156	W
Total Estimated MOSFET Power Loss	P_FET =	1.307	W
Recommended Clamping Voltage on Drain	V_DRAINclamp =	635.0	V

OUTPUT DIODE, D_OUT
Forward Voltage Drop of Output Rectifier, V_F =	V_F =	1.00	V	User Input
Minimum Required Blocking Voltage Rating	V_{DOUT_blocking} =	58.9	V
Required Minimum Average Rectified Output Current	I_Dout =	2.608	A
Power Dissipation of D_OUT	P_Dout =	1.556	W

AUXILIARY WINDING DIODE, D_AUX
Auxiliary Rectifier Forward Voltage Drop	V_FA =	1.00	V	User Input
Minimum Required Blocking Voltage Rating	V_{DBIAS_blocking} =	57.5	V

OUTPUT INDUCTOR, L_OUT
DCR of Output Inductor	DCR_Lout =	0	mΩ	User Input

OUTPUT CAPACITOR, C_OUT
Minimum Required C_OUT Without Opto-Coupled FeedBack	C_{OUT_no_opto} =	2600.0	µF	The importance of using opto feedback should be noted here!
Minimum Required C_OUT Without Opto-Coupled FeedBack	C_{OUT_no_opto} =	2600.0	µF	The importance of using opto feedback should be noted here!
Recommended Minimum Required Output Capacitor With Opto-Coupled FeedBack	C_{OUTrecommended}=	470.0	µF
	C_{OUTrecommended}=	470.0	µF
Actual Output Capacitance Used	C_OUTactual =	470.0	µF	User Input
C_OUT Used in Calculations	C_OUT =	470.0	µF
Required Minimum Ripple Current Rating	I_COUTrms =	2.040	A
Recommended Maximum ESR	ESR_{Coutrecommended} =	12.2	mΩ
Actual ESR of C_OUT Used	ESR_Coutactual =	10.0	mΩ	User Input
ESR Used in Calculations	ESR_Cout =	10.0	mΩ
Resultant Output Voltage Peak to Peak Ripple	V_OUTripple =	67.6	mV

VOLTAGE SENSE DIVIDER, R_VS1, R_VS2
VS Line Sense Run Current, Minimum	I_{VSLrun_min} =	170	µA	Device Parameter
VS Line Sense Run Current, Nominal	I_{VSLrun_nom} =	210	µA	Device Parameter
VS Line Sense Run Current, Maximum	I_{VSLrun_max} =	250	µA	Device Parameter
VS Line Sense Stop Current, Minimum	I_{VSLstop_min} =	60	µA	Device Parameter
VS Line Sense Stop Current, Nominal	I_{VSLstop_nom} =	75	µA	Device Parameter
VS Line Sense Stop Current, Maximum	I_{VSLstop_max} =	90	µA	Device Parameter
Recommended Resistor Value for Minimum Start Up Line Voltage	R_{VS1recommended} =	86.6	kΩ
	R_{VS1recommended} =	86.6	kΩ
Actual Resistor Value Used for Minimum Start Up Line Voltage	R_VS1actual =	86.6	kΩ	User Input
	R_VS1actual =	86.6	kΩ	User Input
R_VS1 Value Used in Calculations	R_VS1 =	86.6	kΩ
Resultant Turn On Voltage, Minimum	V_TURNONmin =	588.9	VDC
Resultant Turn On Voltage, Nominal	V_TURNONnom =	727.4	VDC
Resultant Turn On Voltage, Maximum	V_TURNONmax =	866.0	VDC
Resultant Input Brown Out Voltage, Minimum	V_BROWNOUTmin =	212.8	VDC
Resultant Input Brown Out Voltage, Nominal	V_BROWNOUTnom =	264.8	VDC
Resultant Input Brown Out Voltage, Maximum	V_BROWNOUTmax =	316.8	VDC
Internal VS Over Voltage Threshold, Minimum	V_OVPmin =	4.45	V	Device Parameter
Internal VS Over Voltage Threshold, Nominal	V_OVPnom =	4.65	V	Device Parameter
Internal VS Over Voltage Threshold, Maximum	V_OVPmax =	4.85	V	Device Parameter
Recommended Resistor Value for Desired Output Over Voltage Limit	R_{VS2recommended} =	33.2	kΩ
	R_{VS2recommended} =	33.2	kΩ
Actual Resistor Value Used for Desired Output Over Voltage Limit	R_VS2actual =	33.2	kΩ	User Input
	R_VS2actual =	33.2	kΩ	User Input
R_VS2 Used in Calculations	R_VS2 =	33.2	kΩ
Resultant Output Over Voltage Threshold, Minimum	V_{OUT_OVPmin} =	15.058	V
Resultant Output Over Voltage Threshold, Nominal	V_{OUT_OVPnom} =	15.779	V	Actual Output Over Voltage
Resultant Output Over Voltage Threshold, Maximum	V_{OUT_OVPmax} =	16.501	V

LINE COMPENSATION, R_LC
Line Compensation Current Ratio, Nominal	K_LCnom =	25	A/A	Device Parameter
Total Estimated Current Sense Delay	t_DELAY =	182	ns
Recommended Resistor Value for Line Compensation	R_{LCrecommended} =	3.320	kΩ
Actual Resistor Value Used for Line Compensation	R_LCactual =	3.320	kΩ	User Input
R_LC Used in Calculations	R_LC =	3.320	kΩ
Result of R_LC selection	Output Constant Current will have minimal deviation over input line voltage range.
Result of R_LC selection

VDD CAPACITOR, C_VDD
Device Supply Current During Run Mode, Maximum	I_RUNmax =	2.40	mA	Device Parameter
VDD_ON Voltage, Maximum	VDD_ONmax =	24.5	V	Device Parameter
VDD_OFF Voltage, Maximum	VDD_OFFmax =	8.30	V	Device Parameter
Estimated Minimum Switching Frequency at No-Load	f_SWmin =	3.252	kHz
Estimated Over Voltage Charge Duration	t_OV =	2.0	ms
Minimum VDD Capacitor for Start UP	C_VDD1 =	2.200	µF
Minimum VDD Capacitor for Load Transient	C_VDD2 =	1.000	µF
Minimum VDD Capacitor for Target Ripple on VDD	C_VDD3 =	0.680	µF
Recommended Capacitor on VDD	C_{VDDrecommended} =	2.200	µF

OPTO-COUPLED FEEDBACK
Reference Voltage of TL431 Shunt Regulator	VREF₄₃₁ =	2.495	V	User Input
Shunt Regulator Reference Input Current, Maximum	I_REF431 =	4	µA	User Input
Recommended Bottom Resistor Value for Output Voltage Set Point	R_{FB2recommended} =	44.2	kΩ
	R_{FB2recommended} =	44.2	kΩ
Actual Bottom Resistor Value Used for Output Voltage Set Point	R_FB2actual =	44.2	kΩ	User Input
	R_FB2actual =	44.2	kΩ	User Input
R_FB2 Used in Calculations	R_FB2 =	44.2	kΩ
Recommended Top Resistor Value for Output Voltage Set Point	R_{FB1recommended} =	226	kΩ
Recommended Top Resistor Value for Output Voltage Set Point	R_{FB1recommended} =	226	kΩ
Actual Top Resistor Value Used for Output Voltage Set Point	R_FB1actual =	226	kΩ	User Input
Actual Top Resistor Value Used for Output Voltage Set Point	R_FB1actual =	226	kΩ	User Input
R_FB1 Used in Calculations	R_FB1 =	226.02	kΩ
Resultant Nominal Constant Voltage Output Voltage	V_{OUT_CV} =	15.253	V
Recommended Value For R_FB4	R_{FB4recommended} =	4.02	kΩ
R_TL Used in Calculations	R_TL =	1.5	kΩ

* All reference designators can be found in Schematic and BoM sheet

On the Schematic BOM tab of the UCC28742 calculator it give specs for the MOSFET Q1 as follows:

Q₁	Minimum V_DS Voltage Rating:	2500	V
	Minimum Continuous Current Rating:	0.669	A
	Minimum Repetitive Peak Current Rating:	1.784	A
	Power Dissipation:	1.307	W

Does the MOSFET current rating really need to be 669mA cont. and 1.784A peak?

The 1st table shows the primary current will only be:

Peak Primary Current, Maximum, Full Load

I_PPmax =

0.177

Thanks,
Clint

0 Mike O' over 6 years ago in reply to Clint Appling

TI__Guru* 86266 points

Hello,

I believe the tools calculation for peak and RMS current for the FETs is correct.

In regards to not being able to find a FET that will allow valley switching. This device will valley switch if half the resonant period between the primary magnetizing inductance (Lp) and the switch node capacitance Co(tr) is 2 us. Co(tr) < or = 1/((2*3.14*500kHz)^2*Lp. If you can’t find a FET with a low enough Co(tr) you could consider lowering your fmax to increase Lp.

Regards,

0 Clint Appling over 6 years ago in reply to Mike O'

Prodigy 245 points

Hi Mike,

I tried adjusting the frequency do to 70, then 65, then 50, then 10kHz and still no matter the frequency setting and adjustment to the primary inductance and primary/secondary turns ratios, I still get the message that the design will not Valley Switch.

It looks like the only way I can get the design to valley switch is to lower the Output capacitance (Coss) to 10pF or less.

Once the Coss value reaches 11pF or higher, then the design states will not valley switch.

I had originally found a MOSFET that would meet the Coss requirement, but the current rating of the MOSFET was too low.
As I search for higher current, high voltage MOSFETs I am finding them all to have at least 77pF, 100pF, or even higher Coss.

I really need the frequency to be high as possible to keep the Flyback transformer as small as possible to save board space.
Are you aware of any type of 2500V or greater MOSFETs that typically have low Coss?

0 Mike O' over 6 years ago in reply to Clint Appling

TI__Guru* 86266 points

Hello,

Your design is unique and due to your high input voltage and power level. Finding a FET that achieves ZVS may not be possible. The UCC28742 is limited to resonant frequencies below 500 kHz.

If ZVS is required it is possible that this cannot be achieved using the UCC28742. You might consider evaluating a different device to see if it meets your needs. The UCC28600 is a QR fly back controller that might be able to meet your design requirements.

The other option is design a hard switched fly back convter. The UC28C42 could work for this application.

Regards,

Mike

0 Clint Appling over 6 years ago in reply to Mike O'

Prodigy 245 points

Hi Mike,

Can you explain the consequences of not attaining ZVS? Will it result in even higher ringing and overshoot voltages on the UCC28742?

This is not a battery powered application, so lower efficiency will not be an issue, although making sure the parts do not over heat will be.

I tried setting the inputs with the UCC28C42, but apparently something is not working out.

My input voltages are within the specified range 16.5-2000V

0 Clint Appling over 6 years ago in reply to Clint Appling

Prodigy 245 points

0 Mike O' over 6 years ago in reply to Clint Appling

TI__Guru* 86266 points

Hello,

I am not the creator of the Webench tool and this input range seems to wide. However, the device should function in the design range you are trying to achieve but it won't achieve ZVS. It will hard switch.

Please note there is TZTO zero crossing timeout delay of 1.45 to 3.3 us. What This means is if a quarter of your resonant period between Lm the switch node capacitance this device will hard switch. It will still function but no ZVS.

Lastly I see that you posted that the Webench tool should be capable of doing this design because it is in the tools range of 16.5V to 2000V. I think that is in error and I am going to assign this to the Webench team to resolve.

Regards,

0 Mike O' over 6 years ago in reply to Clint Appling

TI__Guru* 86266 points

To the Webench team can you looking this and resolve Clint's issue with the Webench tool? It is not capable of doing his design.
I believe the input range is in error.

Regards,

0 Umayal Ramanathan over 6 years ago in reply to Mike O'

TI__Expert 5820 points

Hi Clint,

I tried creating the design for the condition reported,I am able to create the design.
I have shared the design in link mentioned below.
Shared design link: webench.ti.com/.../SDP.cgi
Could you please let us know if you are able to create the design now.

Thanks and Regards,
Umayal

0 Clint Appling over 6 years ago in reply to Umayal Ramanathan

Prodigy 245 points

So the webench link you posted shows the design must be acceptable.

My question now is though, is my design not acceptable for the UCC28742 which I was last told to use and created a PCB schematic for already?

This is what I was told:

Clint,

I asked about info on designing the bias circuit, while also asking about if LLC topology was recommended for your use case, directly to our UCC25600/high voltage controller team. Here was their response:

First, I think an LLC for 15W is not practical and a flyback converter is an optimal choice.

I also don’t believe the UCC25600 is our best part for this job. This could be done with a UC28742 QR to DCM flyback converter.

The device is easier to use in these low power applications than the UCC25600.

Will WEBENCH allow the same input votage range and output voltage level on the UCC28742 ?

I have been using the UCC28742 Design Calculator Tool that was recommended to me.
The Design calculator is where I came across the not valley switching operation, and was trying either solve it or make sure the design would operate normally even without attaining valley switching.

Is the Design calculator equivalent to the WEBENCH tool?

When I try to use the WEBENCH tool with the UCC28742 it pre-fills the inputs and will not let me select DC.
Also, the voltage range input is very low (400V max) when the Design Calculator lets me enter 1100VDC with no issues?

If possible to make the circuit operate ok, I would like to remain using the UCC28742 since I have already got the parts into our PCB software.

I do not want to have to change to a different IC chip unless there is a significant improvement in operation or the UCC28742 will not be possible.
Will the UCC28C42DRG4 operate about the same as the UCC28742 or would there be any significant improvement? Would valley switching be attainable with the UCC28C42DRG4 or not?

Regards,
Clint

0 Clint Appling over 6 years ago in reply to Mike O'

Prodigy 245 points

Here is my Schematic currently.
I believe I may need to adjust some values though.

Regards,
Clint

0 Harish K R over 6 years ago in reply to Clint Appling

TI__Intellectual 2320 points

Hi Clint,

Since you had mentioned that you were facing issue creating design for UCC28C42 in WEBENCH, we just wanted to check if you were able to resolve issue in WEBENCH. Apologies for the confusion created.
Regarding you application circuit design, kindly follow the recommendation as mentioned by Mike and let us know in case of any queries.

Thanks & Regards,
Harish

0 Eric Faraci over 6 years ago in reply to Clint Appling

TI__Expert 8210 points

Clint

UCC28C42 is a fixed frequency controller. It does not have valley switching.

The reason the tool is recommending that the aux winding is that the VDD voltage is recommended to be higher. For example in the electrical characteristics VDD is 25V. The typical schematic that the excel calculator is designing for has a separate aux winding from output (since they are on opposite sides of the transformer) so it calculates an optimal turns ratio. In order to achieve galvanic isolation the aux winding and output winding have to be completely separate from each other. There's nothing in this design so far that would prevent valley switching.

Best Regards,
Eric

Power management

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WEBENCH® Tools/UCC28742: MOSFET selection for Valley Switching