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UC2845: UC2845 will generate very low duty cycle PWM pulse when ISENSE reaches 1V

Howard Zou
Genius 12365 points
Part Number: UC2845

Hi,

my customer's schematic is as below:

At normal condition, V1_DRV is 12V, the output VC_boost to VE_boost is 25V. The OUT pin duty cycle is 42%.

They will observe that the duty cycle of OUT pin decrease to a very low level and VC_boost to VE_boost will decrease to around 10V, since the voltage is given to gate driver which will drive the gate side of the IGBT, low voltage will let the IGBT be in saturation region and burnt out. So it's a serious problem.

Blue line is (VC_boost - VE_boost).

Out pin voltage waveform when ISENSE is 1V is the yellow line below, it's a very low duty cycle pulse, which result in the drop of the output winding voltage.

The question is:

1. why the OUT pin's duty cycle is very low when ISENSE=1V(which represents over current).

2. Are there any way to achieve over current protection(turn off OUT pin) within UC2845?

  • 0
    TI__Mastermind 31580 points

    Hi Howard,

    When the voltage on the CS pin hits or exceeds 1V the PWM duty cycle will be terminated. This will reduce the duty cycle and the output voltage will drop because the duty cycle is now reduced. This is normal operation during an over current event.

    Another way to have overcurrent protection is to have an external circuit that pulls the COMP pin low and turns off the gate drive completely and then the output voltage will collapse.

    Why is the power supply going into current limit ?

    Regards

    Peter

  • 0 in reply to Peter Meaney
    TI__Genius 12365 points

    But we observed that the PWM is not terminated, but duty cycle decreased to a very low value as shown in the picture I posted when CS hit 1V.

    The power supply is not going into current limit, but VCC goes high and causing CS pin to hit 1V.

  • 0 in reply to Howard Zou
    TI__Mastermind 31580 points

    Hi Howard,

    The duty cycle cannot go to zero during overload. The PWM pulse will terminate only when the 1V limit is reached on the CS pin so there will be a short duty cycle during the over current condition.

    If the customer needs shutdown or latch off during over current them they have to add an external circuit.

    Vcc may go higher during over load due to increased voltage on the aux winding, the customer can try and add a 10R resistor in series with the aux winding feeding CD55, 100nF cap. The series resistor will help reduce the peak voltage on the cap and Vcc pin.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    TI__Genius 12365 points

    Peter,

    1. you mean that PWM will terminate(turn off) at this cycle when CS reaches 1V, and it will start again in the next cycle?

    2. Do we have reference circuit to achieve shutdown during over current within UC2845?

  • 0 in reply to Howard Zou
    TI__Mastermind 31580 points

    Hi Howard

    1. Yes that is correct, once the CS signal hits the 1V threshold the PWM is cycle is terminated and then the PWM cycle starts again after the RC pin is discharged internally.

    2. This depends on what behavior  the customer want to see in the over current condition,

    - shutdown and restart of PSU after power cycle, so like a latch off situation.

    - shutdown and automatic restart after some time, like hiccup mode.

    Regards

    Peter

  • 0 in reply to Howard Zou
    TI__Mastermind 31580 points

    Hi Howard,

    The VCC rail must continue to be present for this circuit to latch off.

    I have put together this circuit which uses an external comparator to pull the COMP pin low when the CS signal exceeds the threshold set on the non-inverting pin of the comparartor.

    The component values may need to be changed to work on the PCB due to noise and speed of response.

    Regards

    Peter