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Problem with TPS40200

kapil sharma62581
Prodigy 20 points
Other Parts Discussed in Thread: TPS40200, TPS40210

Hi, I have used TPS40200 switcher IC for my power supply requirement. Following is the configuration:

Vinput = 7.5V to 6.0V

Vout = 5V @ 150mA Max

With the TPS output of 5V, I am driving a transformer based isolated power supply section for analog section, which is rated as below: +/-5V analog @ 130mA max. The supply to analog section is controlled through digital section, which enables the supply to analog section at desired and fixed interval.

The problem I have observed is: The moment i turn on the supply to analog section, TPS40200 goes into shut down mode permamnently. This happens only if the load is connected on +/-5V analog section. If no load connected, it works well. I have tried with a load of as low as 1mA. I have checked the current through the transformer during startup, it is shooting up for about 3-5uSec duration.

I have tried with replacing the current sense resistor of 20mOhm at present to a short, but the result was still the same.

Please suggest a possible solution to this problem.

I have tried a solution, and it is as below in the SCH. I would also like to know the consequences of this option, and any other suggestion possible.

 

 

In the above picture, R76 before L2 is added to limit the current to the supply section. Though it is dissipative, but it still gives the desired output, at the cost of efficiency. By doing so, my board works well. Please suggest pros and cons of using such configuration.

Also, please tell me, what will be the difference if I put the resistor before L2 inductor/ after L2 and before capacitor C2/ after C2 and C40, and before the power supply plane and feedback to the switcher IC?

 

Awaiting your response at the earliest.

 

Thanks,

Kapil

  • 0
    TI__Guru 56965 points

    Kapil,

    I'm not sure why you would be turning the TPS40200 controller off "permimantly" unless the circuit you're using was generating a load that was triggering the OCP fault on restart.  You can confirm that is the case by observing the voltage on the SS pin (C5).  If the IC is repeatled triggering OCP, the C5 voltage should have a ramp up to about 1V and then a much slower ramp down to 150mV.

    In the above picture, R76 before L2 is added to limit the current to the supply section. Though it is dissipative, but it still gives the desired output, at the cost of efficiency. By doing so, my board works well. Please suggest pros and cons of using such configuration.

    Aside from the resistor dissipating 256mW of power at 160mA load current, the resistor will drop 1.6V @ 160mA of current.  That drop will not allow regulation from a 6V input to a 5V output, so immediately you may need to consider a smaller resistor.  Other than that, the resistor presents a huge damping effect to the L-C filter and moderating effect to the power-stage resonance and loop response.

    It might be possible that the loop gain is too high and the TPS40210 controller's response to the sudden load of the digitally controlled -5V inverter is resulting in OCP shut-down.  If you want to look into that, monitor the Output Voltage (TP29) and the control voltage (TP35) when the digitial control turns on.  If the voltage at TP35 is shooting up, the TPS40200 is responding to the drop in the output voltage and sourcing a large amount of current to the output capacitor to recharge it.  If that is the problem, I would recommend reviewing the loop compensation design.

    Also, please tell me, what will be the difference if I put the resistor before L2 inductor/ after L2 and before capacitor C2

    There is no difference between putting the resistor on the switching node side or the output side of the inductor as long as it is between the switching node and the output capacitors.

    after C2 and C40, and before the power supply plane and feedback to the switcher IC?

    Moving the resistor after the output capacitors does effect the system.  This will cause the output voltage to drop much faster on a load change as the current drawn from the capacitors & the switch mode power supply need to pass through the resistor to the output.  In the earlier positions (before the capacitors) the capacitors provided hold-up of the output while the TPS40200 controller reacted to the drop in output voltage and increased the switching duty cycle to compensate.

  • 0 in reply to Peter James Miller
    Prodigy 20 points

    Hi Peter,

     

    Can you please comment on the position of the capacitor C36? I have been having issue with the position and connection of that capacitor. Please suggest.

    Further,

    Also, please tell me, what will be the difference if I put the resistor before L2 inductor/ after L2 and before capacitor C2

    There is no difference between putting the resistor on the switching node side or the output side of the inductor as long as it is between the switching node and the output capacitors.

    Kapil: If I place the resistor after the inductor and before the capacitor, the circuit doesn't work. And, yes, it is always going into OCP mode, and doesn't come out of it. I have no clue why is it happening. It didn't happen on the boards I made made earlier, around 6 of them. And, needless mention, I have checked everything else which is same as my previous design only, and it works with older pwoer supply.

    Thanks,

    Kapil 

  • 0 in reply to kapil sharma62581
    TI__Guru 56965 points

    Kapil,

     

    Is there any chance we could look at the schematic of the +/- 5V converter so we can analize it's start-up characteristics?  I suspect that what we're seeing is a problem with that converter being in a low input imediance state at start-up.  My understanding is that you're powering a micro-controller with the 5V ouptut of the TPS40200 controller and then using that micro controller to control the switching of a transformer based +/- 5V converter.

    Have you checked on the default state of the MOSFETs in your isolated +/- 5V converter?  If the MOSFET is "ON" when the micro is starting up, it is possible that Load 2 is presenting a low impedance to the TPS40200's output and triggering current limit.

    Alternately, if your micro-controller based converter is not providing a soft-start function, it could be drawing too much current trying to ramp the ouptut voltages too fast.

    Lastly, If the micro-controller is trying to start the isolated converters output when the 5V ouptut is very low, it could be drawing an extremely high current from the TPS40200 based power supply.

    Looking at the schematic of this second power supply should help us determine if any of those are the cause.

    Can you please comment on the position of the capacitor C36? I have been having issue with the position and connection of that capacitor. Please suggest.

    Electrically, you have the capacitor in the right location.  It needs to present a filter to the voltage seen at the current limit resistor.  Physically, the ISNS pin connection should be close to the IC pin and the VIN connection should be connected as close to the current sense resistor as possible.  While the capacitor is self should be located at the IC, a seperate trace with no other conenctions (called a Kelvin connection) should be used from C36 to the VIN terminal of the current sense resistor.

    A similar kelvin connection should be made between the MOSFET source terminal (Q7 oin 4) of the current sense resistor (at the current sense resistor - R60) to the current limit filter resistor (R12)

    Kapil: If I place the resistor after the inductor and before the capacitor, the circuit doesn't work. And, yes, it is always going into OCP mode, and doesn't come out of it. I have no clue why is it happening.

    I'm not sure why swapping the inductor and resistor order isn't working.  In series, there should be no difference between Resistor - Inductor and Inductor - Resistor.  Unless there is an unintended connection, they should react the same to the circuit.

    It didn't happen on the boards I made made earlier, around 6 of them. And, needless mention, I have checked everything else which is same as my previous design only, and it works with older pwoer supply.

    Just for clarification, what has changed from the older design to this design?