# Why Power supply have unreasonable no-load input power consumption?

#### Younes Thabet

Joined Jan 9, 2019
53
Hello all,

I have a SMPS power supply based on the TNY276 PMIC, I did a "No-load input power consumption" test using DMM connected to AC line input to measure current values while varing the input voltage from 85VAC to 265VAC...
I got the following results:
 Input Voltage (VAC)
 Input Current (mA)
 Input Power (mW)
85
 7.07
600.95
1058840
12591125
145101450
16511.21848
18512.42294
20513.652798.25
22514.93352.5
24516.173961.65
26517.484632.2

The results are ridiculous, I don't know why the power supply is consuming this much power when no-load is connected!!! although datasheet of TNY276P promises that a no-load input power (at 230VAC) should not surpass 0.15W !!
See circuit below:

Can someone help me identify where is the problem with this power supply!!?

#### AlbertHall

Joined Jun 4, 2014
11,314
What is the part number of ZNR1?

#### crutschow

Joined Mar 14, 2008
27,225
If something is using 4.6W in your circuit, then it will be getting very hot.
Have you felt the various devices to see which one is?

If not, then your measurements are apparently wrong, perhaps due to a low power factor.
The best way to measure line power is to use a power meter such as a Kill-A-Watt, which takes the power factor into account.

#### AlbertHall

Joined Jun 4, 2014
11,314
Can someone help me identify where is the problem with this power supply!!?
On second look, you have 2 100nF capacitors across the input and they will draw about 19mA at 60Hz input so that may account for the current your DMM shows. However that current will be out of phase with the input voltage so will not constitute a power drain. You will need a power meter which takes account of the phase difference between voltage and current.

#### ronsimpson

Joined Oct 7, 2019
1,232
On second look, you have 2 100nF capacitors across the input and they will draw about 19mA
You could remove the caps for a test or power the supply form a DC bench supply. (just to test how much of the current is from AC out of phase)

#### Younes Thabet

Joined Jan 9, 2019
53
If something is using 4.6W in your circuit, then it will be getting very hot.
Have you felt the various devices to see which one is?

If not, then your measurements are apparently wrong, perhaps due to a low power factor.
The best way to measure line power is to use a power meter such as a Kill-A-Watt, which takes the power factor into account.
Nothing is getting hot, i've checked !!
for measurement i only connected a DMM in series with the neutral of mains...
can you please explain a bit more the method of measurement you suggested ..

#### crutschow

Joined Mar 14, 2008
27,225
So most of the "watts" you are measuring is apparent (reactive power) (current out of phase with the voltage) or VARS .
A capacitor or inductor can draw AC current but will dissipate no significant power.
If you don't understand AC power factor due to reactive elements (inductors and capacitors) then I suggest you read up on that.

A power meter such as the Kill-A-Watt takes the power-factor into account when doing the measurement so you only see the real power displayed.

#### ronsimpson

Joined Oct 7, 2019
1,232
Another idea is to remove "NTC1" and measure the current. This will measure the current in the filter. (capacitor current). My guess is that you will find almost all the current you see is through the 100nF caps.

Another way is to put the current meter across the "NTC1". (or remove the NTC1 and measure the current there) The current reading will then exclude the current in the filter which is not causing "watts".

#### BobaMosfet

Joined Jul 1, 2009
1,776
So most of the "watts" you are measuring is apparent (reactive power) (current out of phase with the voltage) or VARS .
A capacitor or inductor can draw AC current but will dissipate no significant power.
If you don't understand AC power factor due to reactive elements (inductors and capacitors) then I suggest you read up on that.

A power meter such as the Kill-A-Watt takes the power-factor into account when doing the measurement so you only see the real power displayed.
This phenomenon is also called 'Wattless Watts' to help the TP search.

#### Younes Thabet

Joined Jan 9, 2019
53
Another idea is to remove "NTC1" and measure the current. This will measure the current in the filter. (capacitor current). My guess is that you will find almost all the current you see is through the 100nF caps.

Another way is to put the current meter across the "NTC1". (or remove the NTC1 and measure the current there) The current reading will then exclude the current in the filter which is not causing "watts".
If I understood you correctly, I need to remove the ntc and measure current there to find real current that causes watts..which excludes the AC current drawn by filter that doesn't dissipate (reactive power)..
As @crutschow suggested, I will read more on this matter and try testing using your tips ..

Thanks,

#### Ian0

Joined Aug 7, 2020
2,235
D4 (US1M) is the problem.
It should be a diode in series with a zener.

Also, I can't see your transformer phase dots very well, but they look as though they might be wrong.

#### Ian0

Joined Aug 7, 2020
2,235
Have you felt the various devices to see which one is?
Don't feel the live parts! They'll feel more than hot!

#### ronsimpson

Joined Oct 7, 2019
1,232
D4 (US1M) is the problem.
If D4 is what the schematic said, then the IC turns on and builds up the primary current. During the flyback time all the current is in D4 and does not ramp down. Next cycle the primary current is still high and the IC thinks there is a short some where and probably goes into a shutdown because of a short mode. D4 might be hot.

#### Younes Thabet

Joined Jan 9, 2019
53
If D4 is what the schematic said, then the IC turns on and builds up the primary current. During the flyback time all the current is in D4 and does not ramp down. Next cycle the primary current is still high and the IC thinks there is a short some where and probably goes into a shutdown because of a short mode. D4 might be hot.
View attachment 237549
No, D4 doesn't get hot at all. also the 12V output is somehow stable at 12.45V.

#### Younes Thabet

Joined Jan 9, 2019
53
As you guys guessed, the voltage and current aren't in-phase which causes a low power factor ..
I have used a hall effect sensor connected to an oscilloscope to see current been drawn by power supply at 85VAC, 230VAC, and 265VAC and I've got the below signals...
Problem is as you can see I can't figure the phase-shift between voltage and current because of the spikes especially at 85VAC, do I deduce the phase-shift using peek of voltage signal and spike of current signal (green one) or between voltage signal's peek and current filtered signal (2nd order lowpass filter "red signal")!!?
Same with 230VAC/256VAC input!?

#### Attachments

• 173.5 KB Views: 2
• 217.6 KB Views: 3
• 219.2 KB Views: 3

#### Ian0

Joined Aug 7, 2020
2,235
No, D4 doesn't get hot at all. also the 12V output is somehow stable at 12.45V.
In that case, you must have blown it. D4 IS a problem.
So is your transformer phase. You have the secondaries out of phase, so you have a forward converter (which is missing an output inductor) not a flyback.