Dear team,
Could you please help answer below two questions? Thanks!
1. Could you please tell me whether our Bootstrap function can support OUTH remains high for 2 hours? In my understanding, the bootstrap cap needs to be charged during OUTH off time, but I don't see input PWM minimum off time requirement in the specs, so I am not sure whether the C_BST cap can keep high voltage for two hours.
2. Do we have any recommendation for C_BST cap package?
Thanks & Best Regards,
Sherry
Hi Andy,
Thanks for your reply!
My customer has more questions about this charge pump as below,
1. OUTH continues to be at a high level. In order to ensure that MN1 is turned on, does the Charge Pump need to work to maintain it? In my understanding, the charge pump has to work. Cbst will discharge with internal resistors, so the voltage on Cbst will decrease. When the voltage decrease to one value, MN1 can't turn on, right? But this discharge takes a long time, so it can keep OUTH to be high level for two hours.
2. The wiring of MN1 is disconnected in below block diagram, where should it be connected?
3. If we use 2kHz switching frequency. What is the max duty cycle? For example, 80% or even larger?
The customer use case background is as below,
As shown in the figure, our system considers most of the working conditions ASC (Active short Circuit) as a safe state.
1. Generally, the low side SiC FET is in ASC working conditions(S4/S5/S6 OUTH is always high level). Generally, many vehicles will maintain this state as long as the low-voltage KL30 is turned on, and the duration is not limited (may last longer than 2 hours).
In addition, under the failure condition, it will also enter the low side(S4/S5/S6) ASC according to the fault condition, and the duration depends on the working conditions of the whole vehicle (for example, when the vehicle fails on a long downhill, it will also enter the low-side SiC FET ASC protection state).
2. Some customer request that the minimum duration of ASC is 30 minutes(motor with current).
In summary, considering the application conditions of the whole vehicle, S1-S6 of below topology need to maintain a continuous high level state, and the following conditions need to be met when maintaining a continuous high level open state (the maximum operating temperature of the chip)
1: Be able to provide a sustained high level to turn on the SiC
2: It cannot cause the driver chip to fail.
Thanks & Best Regards,
Sherry
Hi Sherry,
Andy is OoO on international travel; I will try to help address your questions while he's out.
1. OUTH continues to be at a high level. In order to ensure that MN1 is turned on, does the Charge Pump need to work to maintain it? In my understanding, the charge pump has to work. Cbst will discharge with internal resistors, so the voltage on Cbst will decrease. When the voltage decrease to one value, MN1 can't turn on, right? But this discharge takes a long time, so it can keep OUTH to be high level for two hours.
As stated in the datasheet, "for slow switching frequencies at high duty cycles the external capacitor may not be able to charge enough during the OUTH off time to supply the gate drive for the entire on-time. In these conditions, the charge pump circuit is used to hold the voltage across the bootstrap capacitor." In a normal bootstrap configuration the bootstrap capacitor will discharge, but with the integrated charge pump of the UCC5870-Q1, we don't have that problem.
2. The wiring of MN1 is disconnected in below block diagram, where should it be connected?
It should be tied to VCC2, which is the source of the OUTH current and high gate voltage.
3. If we use 2kHz switching frequency. What is the max duty cycle? For example, 80% or even larger?
The device supports 100% duty cycle thanks to the charge pump.
Regards,
Audrey
