Original question:
High-voltage solid-state relay: TIDA-050063: Selecting the inductor
Hi Team,
Is it suitable that i put a heating resistor between V_Cap and -HV for battery pre-heating?
Will the switching frequency too low to keep a constant current?
The battery voltage around 120V and output current for heater around 7A
if it works, how can i choose the inductor?
Thank you
Hi Tilden,
We are planning to place 180uF or 560uF in parallel to heating resistor.
i have used PspiceTI run sim with circuit like TIDA-050063 but with 180uF// 10 ohms heating resistor.
And the current pass the resistor was around 3A
If i want to create 400W heating power and output current below 7A
How to choose the resistors and inductor?
Thank you.
Hello Jianmin,
I'm also seeing the issue when simulating this circuit in Pspice, still trying to understand what is happening. But it looks like the DC-link capacitor (56uH) is not charging up to the V_battery voltage (120-V) once we add the heating resistor (10-Ω) in parallel and the pre-charge becomes stuck. Since the DC-link capacitor voltage cannot charge up further but is not yet up to V_battery voltage (120-V), it becomes stuck at the end.. I need some more time to think about this, will try to respond by Friday (17 Nov).
Best regards,
Tilden Chen
Solid State Relays | Applications Engineer
Hello Jianmin,
Thanks for your patience here, I understand some of the issue now. The problem was that the average current sent through the inductor was less than the current drawn through the 10-ohm heating resistor. So earlier, we were sending an average of 4-A through the inductor (blue). Once V_cap = 40-V (pink), the current through the 10-ohm heating resistor (red) becomes equal to the current coming in (blue), so the capacitor cannot charge up further and becomes stuck at around 40-V.
We can fix this by making sure I_AVG ≥ I_charge + I_heat.
Let's say we want to charge 180-uF to 120-V in 100 ms.
I'm not sure why the charge time here is 16 ms, need to look into this further. I also don't understand all the limiting factors in your design (for example if the heating resistor needs to be smaller, if the inductor needs to be smaller, etc), so I've attached the Pspice project for you to experiment with.
TIDA-050063_Jianmin.zip
Best regards,
Tilden Chen
Solid State Relays | Applications Engineer
Hello Jianmin,
I'm still trying to understand how the heating resistor affects the pre-charge time. It definitely increases the time compared to pre-charge with no heating resistor, but I'm not sure what the best way is to estimate this increase. Looking at the simulation when I probe the current into the inductor, capacitor, and resistor, I get the following results:
It looks like current through the heating resistor (red) starts at 0-A and settles to 3.3-A. So let's linearize this and say I_heat_avg = 1.67-A.
The inductor current we set to begin at 3.75-A and settles to 3.33-A. So let's linearize this and say I_L_avg = 3.50-A.
The average current to the inductor must be the difference between the two. So I_cap_avg = 3.5-1.67 = 1.83-A.
t = Q/I
t = 0.0216 C / 1.83-A
t = 11.8 ms (this might be okay for an estimate considering the decay and rise are nonlinear. Can use this estimation for finding initial passive component values and need to simulate for better accuracy at the end)
That means I_heat will dominant in "I_AVG ≥ I_charge + I_heat", and the DC-link capacitance will not significantly affect the I_AVG, right?"
We aim to set R_heat between 10- and 20-ohm
Our target is to maintain I_AVG at 5±1A without DC-link capacitor voltage reach V_battery voltage( means NMOS keep switching)
In this situation, do you have any recommendations for selecting the appropriate inductor and setting the values for Rbot, Rtop, and Rhys?
Are there any specific considerations, besides the I-sat of the inductor, that I should keep in mind when choosing the inductor?
I'll do an example and hopefully that can answer most of your questions here.
Disclaimer: I am not an expert on inductors. Looking at these inductors, it looks like lower inductance --> can handle more current. I think at minimum Irms > I_AVG (5-A) and Isat > I_AVG_max (6-A). So the largest inductor we can use from this list is 390-uH.
However, we are also limited by the TPSI3052-Q1 switching frequency. Smaller inductance --> needs higher switching frequency. For the FETs used in the design guide, the TPSI3052-Q1 can switch for a maximum of 67-kHz. Let's select the 330-uH inductor for some margin and calculate the maximum switching frequency here.
F_MAX = 120 / (4*330uH*2A)
F_MAX = 45-kHz (Less than 67-kHz, so the 330-uH inductor will work here)
I_AVG = 5-A ± 1 (so 4-A to 6-A). We have a 100-mΩ sense resistor, so the voltage thresholds are 0.4-V and 0.6-V.
If we want to settle at 5-A while still switching, we need to select a resistor to draw 5-A before V_cap reaches 120-V.
t = Q/I
t = 0.018 C / 2.5-A
t = 7.2-ms
Please let me know if you have further questions.
Best regards,
Tilden Chen
Solid State Relays | Applications Engineer
