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H-bridge - MOSFET driver

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I am trying to implement an H-bridge with the MIC5021 for the high side of both legs and an MIC5020 for the low side of both legs.

I have used the MIC5020 in other designs and it seems to be working great. This is the first time I have used the MIC5021 and I am not getting the boost voltage needed to fully turn on the high side MOSFET.

When the train of pulses starts, I see one large pulse at close to the correct voltage and over the next few pulses (3-4) it diminishes to system voltage (in this case 24 V). Initially I had a 0.1 μF capacitor which I later realized was way too big and possibly damaging the MIC5021. I downsized this to 0.01 μF and I still am seeing similar results. I have ordered 2.7 nF capacitors to try next and I am currently waiting for them to arrive.

In the datasheet for the MIC5021 I see the boost capacitor tied to system voltage in one diagram and the source of the MOSFET being driven in another. Which diagram is correct/better? Is there an advantage of one configuration over the other? Ultimately I need to get the boost capacitor generating the proper voltage to drive the MOSFET fully ON as it is not doing it right now. What can I do to address this?

In the attached image the system voltage is 24 V. Here are the signals:

Yellow: High-side signal from controller
Blue: Low-side signal from controller
Red: @ Low-side gate
Green: @ High-side gate

1675072297191.png
 

Hi,

for system volateges from +20 V to +36 v they recommend the connection of the boost capacitor towrads VDD.

The front page of the datasheet states "The MIC5021 high-side MOSFET driver is designed to operate at frequencies up to 100 kHz (5 kHz PWM for 2% to 100% duty cycle) ... ". With a period of ~32 µs your switchin frequency is about 31.25 kHz, where your on-time is about 4 µs, leading to a duty cucle of ~8 %. So your switching frequency might be too high. Which MOSFET's are you using?

As @KlausST pointed out, a schematic would be quite helpful.

BR
 

Hi,

for system volateges from +20 V to +36 v they recommend the connection of the boost capacitor towrads VDD.

The front page of the datasheet states "The MIC5021 high-side MOSFET driver is designed to operate at frequencies up to 100 kHz (5 kHz PWM for 2% to 100% duty cycle) ... ". With a period of ~32 µs your switchin frequency is about 31.25 kHz, where your on-time is about 4 µs, leading to a duty cucle of ~8 %. So your switching frequency might be too high. Which MOSFET's are you using?

As @KlausST pointed out, a schematic would be quite helpful.

BR
I am confused a little. When it is mentioned below as
5 kHz PWM for 2% to 100% duty cycle
What is the PWM for 100% duty cycle?

Also, could you please tell me how you arrived at the values of 32us, 31.25kHz and so on?
 

Hi,

I came to the same results, just by looking at your scope picture.

Why is it so hard to post a schematic? You just waste time.

Klaus
I am retriving the schematic from my colleague. Please wait. Also, could you please explain What is the PWM for 100% duty cycle?
 

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