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20th April 2017, 15:34 #1
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my Circuit drawing current with no load
Hi folks,
I build the attached circuit (it is a simple voltage doubler, Vou=2Vin). I used ring oscillator to generate 200kHz frequency (Phi1, Phi2 in the circuit) these two signals are nonoverlapping. I got a perfect output voltage as expected. However, when I measure the input current, it seems my circuit always drawing current of around and it keeps increasing when I increase the input voltage. For example, with 0.5V input, the current is 17.6mA. This circuit is built for very low input voltage (ranges 01V)
I thought that the capacitors draw the current in charging, but they should hold the charge in steady state. I am using 2.2uF ceramic cap, I measured the voltage across the cap and it gave me almost dc voltage which I expect ideally. So, the caps are balanced and once they charged they keep their voltage!
Any ideas to share about this problem?
Also, one simple question, how can I measure the input impedance of my circuit? my sense this impedance is changing depends on the switching, Vin and other factors. Can I just measure the input with multimeter while it is not powered?!

21st April 2017, 04:04 #2
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Re: my Circuit drawing current with no load
leaky caps. Also what are your switches, if the timing over laps you'll get lose, but also can you be leaking from the switch to ground.
I'm assuming your using a analog switch. what are the rails of the switch in relation to your Vin?

21st April 2017, 07:16 #3
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Re: my Circuit drawing current with no load
Hi,
The oscillator needs power to run.
And it drives capacitive loads: traces, mosfet gate capacitance with it's miller capacitance.
All the charged energy needs to be dissipated as heat > causing supply current
Klaus

21st April 2017, 07:16

23rd April 2017, 01:04 #4
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Re: my Circuit drawing current with no load
The oscillator is powered by a USB cable connected to my laptop.
   Updated   
I'm using nMos switches. What do mean by analog switches?
Regarding the voltage over the switch, they are around 23 multiples of Vin.
The clocks are nonoverlapped and there is no shoot through states, that what made me questioning about drawing current in steady state

23rd April 2017, 01:32 #5
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Re: my Circuit drawing current with no load
Hi,
Maybe it's time to show the real, complete circuit.
What ceramics type are the capacitors?
Klaus

24th April 2017, 02:10 #6
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Re: my Circuit drawing current with no load
Hi Klaus,
I am ceramic caps from TKD, and small signal switches from infineon: their datasheets are attached. regarding the circuit, I am using the same as shown in previous pic with addition to ring oscillator and nonoverlap clocks generator using 74HC04 hex inverter IC and 74HC00 NAND gates IC. They are all attached.
   Updated   
forgot to say, my ring oscillator generates 200kHz clock to drive the MOSFETS in the converter

24th April 2017, 05:31 #7
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Re: my Circuit drawing current with no load
Hi,
With 1n at the AND gate outputs I expect rather slow rise and fall rate.
Can you show both clocks in one scope picture..
I can"t see how you ensure "non overlap" clocks.
Klaus

28th April 2017, 02:29 #8
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Re: my Circuit drawing current with no load
I am sorry Klaus for being late, just go a travel on the way!.
the clocks are attached. after The NANDs and without adding the 1nF caps, there was overlapping which is some I don't want. So, I placed the 1nF to slow down (add delays) to both of them and by that I got enough time for switching off (Sa transistors) before Sb transistors turn on.

28th April 2017, 03:26 #9
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Re: my Circuit drawing current with no load
You are using three gates from a hex inverter to make an oscillator. That part is fine.
You have not used an external R for adding a time constant; you are using internal delays and internal resistance to control the time constant. I do not like that.
At 200kHz that may not be the best idea.

28th April 2017, 03:26

28th April 2017, 04:31 #10
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Re: my Circuit drawing current with no load
Thanks c_mitra for your input. don't you think adding C is enough?
because my thinking to increase the delay, I need to increase R or C or both. So, I decided to increase C and also I am getting a nice 200kHz pulse. Would you please share with me why you think it is not good avoiding using R?

28th April 2017, 04:51 #11
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Re: my Circuit drawing current with no load
A capacitor just appears as a short to a voltage pulse the current will be limited by internal circuit within the IC. The logic pins can source or sink only a finite current.
Without an R you do not know the time constant and your control over the circuit function is limited. The capacitor(s) charge and discharge with very large currents (true it is for a very short time).
Even a 1 Ohm resistor will improve both reliability and performance.

28th April 2017, 14:25 #12
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28th April 2017, 15:03 #13
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29th April 2017, 07:20 #14
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Re: my Circuit drawing current with no load
Hi,
The series R should be at least 5 times the expected source impedance of the driver...to give reliable and calculable results.
Klaus
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29th April 2017, 10:52 #15
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Re: my Circuit drawing current with no load
If you calculate the energy stored in the ring oscillator capacitors and multiply with 200 kHz, you'll know where the unwanted quiescent current comes from. A reasonable design would reduce the capacitor values at least by factor 100 and add respective series resistors to the gate outputs.
There will be still some quiescent current caused by the unavoidable Cgs and Cds of SC power switches.

29th April 2017, 15:14 #16
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29th April 2017, 16:29 #17
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Re: my Circuit drawing current with no load
Thanks all for your input. I think you focused on the Ring oscillator issue and forget about the main problem. The ring oscillator and control circuit is totally powered from a USB (Not connected to the power supply that I am facing issue with. The main problem in my circuit is that, when I use the power supply to act as an input voltage source to my circuit, the circuit seems to draw high current. The control circuit has nothing to do with the power supply.

29th April 2017, 17:01 #18
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Re: my Circuit drawing current with no load
Thanks for clarifying. Regarding excessive quiescent current of charge pump only, you need to check the gate voltage waveforms. I guess you have partly overlapping clocks.
I would expect an input current in the 100 µA range with BSS816, 0.5V Vin and 5V clocks.
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29th April 2017, 17:35 #19
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Re: my Circuit drawing current with no load
True.
But I am not happy with your dead times. The flipflops are usually very fast (rise and fall times) and the mosfets are really not that fast. The capacitors you use slow down the rise and fall times and the square pulses become trapezoidal. Because the graph is symmetric, fall part of one will intersect with the rise part of another around the middle (roughly speaking). That is the part I do not like.
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29th April 2017, 17:35

29th April 2017, 21:01 #20
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Re: my Circuit drawing current with no load
ok, I will modify the oscillator circuit as you suggested to increase the dead time period, it seems to be the problem based on your input.
Thanks all for your nice and helpful comments, I really appreciate it.
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