Variable Voltage source using PWM and Mosfets.

[Ashkar]

Specifications.
1 Amp continous Current.
Voltage range of 0 to Vdd
Control signal type PWM(Microcontroller output)
Current sense to limit voltage output .

Voltage is supplied from 2 lithium batteries connected in series to give 3.6+3.6v Dc.

 

[horace1]

Microcontrollers usually have a range of apploication notes, e.g. Microchip
**broken link removed**

have a look in the list for power supplies or PWM

 

[Ashkar]

I am unable to find any useful source .
Kindly help a bit more.

 

[horace1]

if you do a web search for "variable voltage source using pwm" or "variable voltage source using pwm technique" it will give a number of links

 

[Ashkar]

My problem is, I want to give PWM wave form from a 5v logic and my maximum available supply is 6.5 or 6
SO I need to know,does irf 540 mosfets get fully on using my available voltage.

 

[Audioguru]

My problem is, I want to give PWM wave form from a 5v logic and my maximum available supply is 6.5 or 6
SO I need to know,does irf 540 mosfets get fully on using my available voltage.

A lithium rechargeable battery cell is 4.2V when fully charged and is 3.2V when the load should be disconnected. Then your supply with 2 cells is from 6.4V to 8.4V.
I do not know if your logic high voltage is 3.5V or 5V.

Why don't you look at the datasheet for the IRF540 Mosfet? It says it is fully turned on when its Vgs is 10V. Its current is only 0.25mA when Vgs is 2V to 4V (its threshold voltage). Use an IRL540 logic level Mosfet instead which can fully turn on with 4V or 5V.
The graphs on the datasheets show a "typical" device but the one you buy might be a minimum spec device. The written specs are guaranteed for minimum to maximum devices.

 

[Ashkar]

I understand that IRL540 would be a great mosfet to use insted of IRF540,
So basically I will turn the mosfet easily using my mcu 5v logic supply.I have a situation also,the pwm frequency is about 20khz would a direct connection to mosfet will turn the mosfet that quickly @20khz.

 

[D.A.(Tony)Stewart]

Ashkar, follow audioguru's line of thinking for voltage range and problems with choice of FET.

My advise if you wish to become a design pro, start with design specs, as audioguru's had started... Then dont choose parts until after factors are considered such as battery C rate of charge, charge balance requirement {preferred}, mode control: startup test, CC mode,CV mode, Charge complete detect& cutoff, charge incomplete & fail.

Look at other charger specs then write your own specs in a table... Don't start designing until you know what you want and know what to expect!
has prerequisite battery theory & charger design background and reasons to prevent premature battery failure.

 

[Audioguru]

A Mosfet has a high gate-source capacitance that is increased by the dynamic capacitance from the drain to the gate. At 20kHz it takes a fairly high current from the signal source to quickly charge and discharge the capacitance. Calculate it to see if the signal source can supply enough current to drive the gate at 20kHz. If it cannot then add complementary emitter followers as a buffer to increase the current.

 

[Ashkar]

An off the topic question,
Asuming that I have 2 shunts in which one is constant ie. 1 milli Ohms and other is variable from 1 miliohms to 1ohms.

Now I have a constant current of 1amps on the source side which is dc.

So according to the formula P=V*I the value of constant shunt should be
R=V/I
0.001Ohms=V/1amp
V= 0.001 volts
Now using power formula P=1A*0.001 V =0.001 watts?
Is this right and if I change the value of the variable resistance to say 1 Ohms then
R=V/I
1ohm=v/1amp
V=1volt

P=v*I
P=1*1
P=1watts
So I think according to the formula that if we use as the value of resistance decreases the power dicipation also decreases.
But how do I know to select the wattage value of the shunt it self.
Because the value of my shunt has to be stable.
Kindly clear my doubt on these parameters.
Thanks.

 

[Audioguru]

A resistor dissipating its maximum power rating is extremely hot and if not out in the open air will get hotter and fail. When the resistor is extremely hot then the heat might burn somebody or melt plastic capacitors or wire insulation nearby. So pick a power rating that is 2 or 3 times higher.

If your shunt resistor is made by a competent name-brand manufacturer then it will be accurate and reliable. But don't buy from ebay a cheap Chinese resistor made by an unknown company.

 

[Ashkar]

I got your explanation,But I am confused due to the above stated calculations,kindly help me out.

 

[andre_luis]

R=V/I
0.001Ohms=V/1amp
V= 0.001 volts

It is not recommended to work with sensed voltage values so small, due its amplitude may lie in the same magnitude of the noise scattered along the board. You must select some higher shunt device so that meets the power constraints, as suggested by Audioguru.

 

[Audioguru]

I do not think a resistor with a value as low as 0.001 ohms is available. It would have wires thicker than my body.
My expensive Fluke multimeter uses 0.3 ohms to measure up to 10A accurately.

 

[Ashkar]

0.2 Micro Ohms will be good.
I mistakenly wrote that the value of shunt should be 0.001.
I opt to use 0.2ohms shunts.
Would that be good.
Now please tell me that to pass a current of 1amps what wattage of shunt has to be used in this design.

 

[andre_luis]

To take a more accurate answer it is necessary to know the current waveform, or circuit topology.

 

[Ashkar]

Current is dc type generated using pwm at 20khz
The circuit is basically a n channel fet with drain connected to a variable resistor and a shunt connected to ground.

 

[assu]

A Mosfet has a high gate-source capacitance that is increased by the dynamic capacitance from the drain to the gate. At 20kHz it takes a fairly high current from the signal source to quickly charge and discharge the capacitance. Calculate it to see if the signal source can supply enough current to drive the gate at 20kHz. If it cannot then add complementary emitter followers as a buffer to increase the current.

could you please tell, if the signal source provides upto 25mAmp max,would that be enough? or can link to some formulation for the calculation.
For learning purpose

 

[Audioguru]

I mistakenly wrote that the value of shunt should be 0.001.
I opt to use 0.2ohms shunts.
Now please tell me that to pass a current of 1amps what wattage of shunt has to be used in this design.

Don't you know the simple formula to calculate power? (I squared) x R. So (1A x 1A) x 0.2 ohms= 0.2W. Use a 1/4W or 1/2W resistor.
Ohm's Law is used to calculate the amount of voltage dropped by the shunt. I x R. So 1A x 0.2 ohms= 0.2V.