LM5046: UNABLE TO OBTAIN HIGH SIDE OUTPUTS

Aneesh R

Part Number: LM5046

Hi everyone,

I am currently working with the IC LM5046MH/NOPB and trying to achieve a phase shifted full bridge operation. I am trying to generate the phase shifted PWM pulses for the switching MOSFETs. I am able to achieve the PWM for low side at terminals LO1 and LO2. At the high side outputs (HS1, HO1, HS2 & HO2), I am not able to observe any PWM.

The PWM on the low side ranges from 0 to 10V. On the high side output terminals, I am just observing a DC voltage of 10V. The parameters of operation are as below:

Vin = 18V
Oscillator Frequency = 50kHz (set by grounding the RT pin with a 100k ohm resistor).

The values observed at the following pins are:

UVLO: 2V
OVP: 1V
REF: 5V
VCC: 9.5V
LO1, LO2: PWM at 50kHz (0 -10V)
HS1, HO1, HS2, HO2: 10V DC

Please Note: The COMP pin was disconnected from the optocoupler, because a current exceeding 1mA flowing into the COMP pin, reduces the duty cycle to 0 as per datasheet. But the issue still persists.

I have checked with the bootstrap capacitors too, the voltage at the BST1 and BST2 pins are also 10V.

I have attached the circuit and the output waveforms for your reference.

Regards,

Aneesh R

The waveform is yellow corresponds to LO1. There is a similar waveform observed for LO2.

The waveform in blue corresponds to HO1. There is a similar waveform observed for HS1, HS2, HO2, BST1 & BST2.

11 months ago

0 Mike O' 11 months ago

TI__Guru* 78646 points

Hello,

This has been received and it under review.

Regards,

0 Mike O' 11 months ago in reply to Mike O'

TI__Guru* 78646 points

Hello,

In your schematic I can not find HS1 and HS2 is labeled pin 23. Could you double check your schematic pin numbering and labels?

Regards,

0 Aneesh R 11 months ago in reply to Mike O'

Prodigy 10 points

Hello Mike,

That has been taken into consideration. The ambiguity is noticed and sorted. But the issue still persists.

Regards,

Aneesh R

0 Aneesh R 11 months ago in reply to Mike O'

Prodigy 10 points

In the schematic, LO1 (pin 15) and SSOFF (pin 23) were interchanged. It was just a numbering error. This issue is taken care of. But the issue remains the same.

0 Mike O' 11 months ago in reply to Aneesh R

TI__Guru* 78646 points

Hello,

Could you send me your updated schematic?

Regards and thanks,

0 Aneesh R 11 months ago in reply to Mike O'

Prodigy 10 points

Hello Mike,

Please find the updated schematic.

Thanks,

Aneesh R

0 Mike O' 11 months ago in reply to Aneesh R

TI__Guru* 78646 points

Hello,

Thank your for the schematic. Your inquiry is under review and I will get back to you shortly.

Regards,

0 Mike O' 11 months ago in reply to Mike O'

TI__Guru* 78646 points

Hello,

Are you testing the design open loop or is connected to a phase shifted full bridge?

From the schematic you sent if it is not connected to full bridge there is no way for the BST1 and BST2 capacitor to charge.

Regards,

0 Aneesh R 11 months ago in reply to Mike O'

Prodigy 10 points

Hello Mike,

As of now, the test is being conducted in open loop condition. In order to give feedback through the optocoupler to the COMP pin, a potentiometer is being used. I have a few questions. It would be really helpful if I get these doubts cleared first.

1. In the datasheet, VCC pin is termed as input, which is described as, "Output of Start-up Regulator". Should an external voltage be given to this pin?
At present, no voltage is given to this pin. The output from VCC is 9.5V.

2. Can this IC be tested in open loop mode, by disconnecting the signal to COMP? Will we still be able to obtain all the four outputs (HO1, HO2, LO1 & LO2)?

3. As per your previous reply, if its not connected to full bridge, but still feedback is given via optocoupler, will BST1 and BST2 capacitor charge?

0 Mike O' 11 months ago in reply to Aneesh R

TI__Guru* 78646 points

Hello,

I think what the issues is that BST1 and BST2 capacitors do not have a place to charge in your open loop circuit. Ground S11 and S12 in your open loop circuit. This will give a reference for the high side drivers and will allow the capacitor to charge for these drivers.

Regards,

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LM5046: UNABLE TO OBTAIN HIGH SIDE OUTPUTS