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How the circuit supply the voltage for the chip TPS92001

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tanhong007

Newbie level 4

Dear all, with the attached is the circuit schematic.

I tried to do the calculation for the power supply part of TPS92001.

I have a few questions about the circuit:

Is the D2 work as a snubber in this circuit?

What is the C6 used for？

How the PNP part works to maintain the supply voltage for the chip TPS92001.

Thank you very much for all your time and efforts!:smile:

D2 is there to ensure current flow goes one way, just in case some accidental connection were to occur.

C6 is not absolutely necessary. It's also there in case of a rogue spike, etc.

In fact the entire network around the PNP transistor 'Q' appears extraneous. It's configured as a constant current source delivered to supply VDD to the IC. (The resistor values are not critical.)

However notice the 15V zener (D7) at the bottom. It regulates voltage so the IC gets a constant 15V.

In this usage a zener can absorb slightly greater or lesser amount of current going through it. Therefore a simple resistor should be sufficient rather than the PNP and associated bias network.

R3 is 1000 ohms. That is probably satisfactory so as to provide enough current to both the IC and the 15V zener.

However it's not clear what voltage the power supply is providing. If it's 120 VDC then R3 and D7 will need higher watt ratings to suit.

It may also be better to use a higher value at C2 and/or C6 so that the IC gets a smooth filtered DC supply.

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I misspoke. Looks like R3 will need to be higher, more like 10K, so as to incorporate R4 as well.

And the power supply appears to do away with a transformer. This could allow fragile components to be exposed to lethal voltage someday if a short happens in the power supply.

Last edited:
tanhong007

tanhong007

Points: 2
C6 - decoupling capacitor
D5, R5, R4, Q -this circuit it's basically a constant current source

tanhong007

tanhong007

Points: 2
D2 is there to ensure current flow goes one way, just in case some accidental connection were to occur.

C6 is not absolutely necessary. It's also there in case of a rogue spike, etc.

In fact the entire network around the PNP transistor 'Q' appears extraneous. It's configured as a constant current source delivered to supply VDD to the IC. (The resistor values are not critical.)

However notice the 15V zener (D7) at the bottom. It regulates voltage so the IC gets a constant 15V.

In this usage a zener can absorb slightly greater or lesser amount of current going through it. Therefore a simple resistor should be sufficient rather than the PNP and associated bias network.

R3 is 1000 ohms. That is probably satisfactory so as to provide enough current to both the IC and the 15V zener.

However it's not clear what voltage the power supply is providing. If it's 120 VDC then R3 and D7 will need higher watt ratings to suit.

It may also be better to use a higher value at C2 and/or C6 so that the IC gets a smooth filtered DC supply.

-----------------------------------------

I misspoke. Looks like R3 will need to be higher, more like 10K, so as to incorporate R4 as well.

And the power supply appears to do away with a transformer. This could allow fragile components to be exposed to lethal voltage someday if a short happens in the power supply.

:grin:
Is that means R3, D5 and R4 formed a voltage regulator sub circuit. The R3 in series with D5 helps to limit the current, and also improve the voltage regulation. Also, the D7, 15V zener diode at the bottom, it regulates the voltage so that TPS92001DG gets a constant 15V as power supply.
So is that possible that I can calculate the power supply voltage for the TPS92001?

---------- Post added at 20:56 ---------- Previous post was at 20:53 ----------

C6 - decoupling capacitor
D5, R5, R4, Q -this circuit it's basically a constant current source
So in case there will be AC signal superimposed on the DC power line, C6 can prevent the powered circuit from those unwanted signals?
But I still don't really understand how this constant current source works.
Thank you very much!

Is that means R3, D5 and R4 formed a voltage regulator sub circuit. The R3 in series with D5 helps to limit the current, and also improve the voltage regulation.

'Sub-circuit' is a suitable description.

However it isn't the mechanism that regulates voltage to the IC. It's true it's a voltage regulator in the sense that the PNP provides constant voltage across R4. The transistor is biased to act this way by the arrangement of R3 and D5.

And why should it provide constant voltage across R4? Since R4 is fixed, then the current is constant.

So the sub-circuit functions as a current source. The purpose of the current source is to limit current to the 15V zener diode.

An explanation of this configuration is at 'circuitspot.com':

Simple Current Source

Also, the D7, 15V zener diode at the bottom, it regulates the voltage so that TPS92001DG gets a constant 15V as power supply.
So is that possible that I can calculate the power supply voltage for the TPS92001?

Yes, that's how it's done.

@BradtheRad: It seems to me, that you are making suggestions to "improve" the circuit without fully understanding it's purpose. That's not very helpful considering the fact, that the original poster rarely understands it. Actually, it's a TI application example and as a starting point, you can assume that it provides at least a suitable way to operate the device.

The reason for using a current source (e.g. instead of a simple resistor) to supply the SMPS chip is quite obvious in my opinion. It simply tries to achieve minimum supply current respectively highest efficiency with a simple circuit. The dimensioning of individual parts matters, and I won't suggest omitting or changing some of them without carefully checking the effect.

...it's a TI application example and as a starting point, you can assume that it provides at least a suitable way to operate the device.

The reason for using a current source (e.g. instead of a simple resistor) to supply the SMPS chip is quite obvious in my opinion. It simply tries to achieve minimum supply current respectively highest efficiency with a simple circuit. The dimensioning of individual parts matters, and I won't suggest omitting or changing some of them without carefully checking the effect.

I admit I didn't realize the TPS92001 is a switch mode power supply IC.

It had occurred to me that the current source ensures the 15V zener will receive stable current. Thus its voltage will stay constant within a minutia of a volt. Thus the IC gets an unchanging supply voltage.

And for all I know this is necessary for proper operation. Or it's intended to allow for wide deviations from spec in production manufacturing.

But now please consider my line of thinking.

As we know, a zener diode is labelled with a stated voltage, yet the tolerance range is several percent.

Therefore both zeners in the schematic can be off by several percent. Meaning that current through the PNP transistor likewise can be off several percent.

Why then should the values for R4 (5.11k) and R5 (301k) be specified so precisely? Are these values truly critical to proper operation? I ask this in view of the fact that zener D7 is really the sole component which ultimately determines the IC supply voltage.

If you ask me this has 'the boss said a simple resistor doesn't look as though there are enough components between the TPS92001 and the 120V supply, so he made me throw in a current limiter' written all over it.

My main point was to emphasize, that the basic 15V supply concept serves a purpose.
Why then should the values for R4 (5.11k) and R5 (301k) be specified so precisely?
They aren't specified precisely, the designer simply prefers values from the E48 resistor series, it's your turn to recognize, that 5 or even 10 percent tolerance are acceptable. My comment refers to the considerations to e.g. omit C6 or increase R3, although both are well considered in my view. The 15V in contrast don't need to be particularly stable, but it's essential to provide a required minimum supply current, continuously.

Some points to consider:
- a variable input voltage, even including dimmer operation of the LED lamp. The optional circuit to translate a "dimmed" input voltage into LED current isn't included in this schematic, but according to the datasheet, the 15V supply is prepared for it.
- R3 is only providing a transient/inrush current limiting
- it's much more effective to place a filter capacitor at the rectified high voltage node (C6) than at the 15V node

Pointing to these details, I don't want to say that the circuit can't be possibly improved. But you should realize it's basic features before.

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