Other Parts Discussed in Thread: LM3478,
Hi Expert,
LM3478-Q1 is used in traction inverter. The topology is Flyback, but it's not isolated design. See the schematic below.
The input voltage is 200~400V, the output is +15/-9V, the max output current is 150mA. Here is the short circuit test waveform:
Vin is 200V, output is +15/-9V, +15V is shorted. The output 15V drop to 0V, but the LM3478 still working, and PWM didn't stop.
In this situation, the output current is 1.5A, so the output diode is too hot to work well.
We read the datasheet, we find there is a 325ns blanking interval to filter the glitch on FB. So I think this is the cause for the case. Here is the screen shot for detail information of short-circuit:
When the short circuit occurs, LM3478-Q1 is powered by the high voltage(Zener outside). In the start of a cycle, the MOSFET still open for 325ns.
Could you help on those questions:
1. We also find that in datasheet: A comparator inside the LM3478Q-Q1 reduces the switching frequency by a factor of 5 and maintains this condition until the short is removed.
In this case, when the output is shorted, we didn't see the switching frequency changed. Is there any way to trigger switching frequency in short circuit? Did this device trigger the short circuit protection?
2. There are 2 current limit value in the datasheet, one is Vsense: current sense threshold voltage, another is Vsc; short-circuit current limited sense voltage.
What's the different between those two parameters? I don't see Vsense in block diagram, how does it works?
Best Regards
Songzhen Guo
