STCMB1 Second Level OCP Protection and Auxiliary Circuit

[MNafi]

Hi,

I am curious that how can ı choose electrolytic capacitor of auxiliary circuit? 

Firstly, I used 10uF electrolytic capacitor at Vcc pin.When I short circuit the output, the HB mosfets has failed.

Then I notice that STCMB1 datasheet mention this topic. According to STCMB1 datasheet, an electrolytic capacitor of at least 22 μF (typ. value) must be connected between the pin and GND. But when ı place 22uF electrolytic capacitor, I'm having trouble with the short circuit test. After a short circuit (about 20s), it causes the LEDs flickering.

Therefore, I used 100uF electrolytic capacitor at Vcc pin.I thought I solved the problem using 100uF but i encountered a new problem. I produced 10 driver and tested them. Almost half of them are undecided about working. The driver which worked yesterday, it does not work today.

[AndresGG]

Hi, how did it go with the power supply design?
I'm designing a power supply with the same chip, but I'm having some problems.

[chriswilson]

It sounds like your capacitor issue is tied to balancing power stability and fault handling. The STCMB1 datasheet recommends at least 22 µF on the Vcc pin to ensure stability, but going higher, like 100 µF, can introduce problems like slower fault recovery or stress on your auxiliary circuit. Here’s what to consider:

1. **Stick to 22 µF**: It’s recommended for a reason. Pair it with a small ceramic cap (e.g., 0.1 µF or 1 µF) for high-frequency transients.
2. **Check the Auxiliary Power Circuit**: Ensure it can handle charging the cap and providing stable power during faults or startup.
3. **PCB Layout**: Ensure good grounding and decoupling to avoid parasitic effects causing instability.
4. **Short Circuit Tests**: The flickering LEDs could mean retry instability or issues with your fault protection timing.

Test under controlled conditions, and compare behavior between working and failing drivers. Fine-tuning might solve the inconsistency! Let me know how it goes.

[AndresGG]

Thank you very much for your recommendations. I designed a power supply but when the load current increases the voltage drops. I tried modifying the ISEN_HB elements to increase the load but under light load the transformer makes noise.