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# [SOLVED]generate isolated dc-dc converter for gate drive

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#### Electro nS

##### Full Member level 6
hello guys

i want to use a mosfet as a high side switch ( instead of a relay)

there is no fast switching (of duty cycle) just 0 or 1 , just ON/OFF each couple of seconds .

to turn on the high side mosfet , you need about 12-15v between gate and source .

but i am using a micro-controller or a key button (digital pin NO PWM), so i need a circuit that can take 3-24v and transform that into isolated 15v . (very low current , 10-50mA)

i have disassembled a solid state relay with similar specification and i cannot on earth under stand the circuit they are using , i have drawn it (if anybody want to investigate i can post)
basically it uses 3coil transformer and an RC + transistor to transform any input DC voltage into AC (+/-2v) then past it to transfomer to create +/-12v signal then rectifies and some staff to drive the mosfet

i am thinking if there is easier way to do it . using 555 timer + transformer ? maybe or a small dc-dc conveter ?
note that most gate drivers are not for this type of applications, since they need switching and separate supply.
i want a reliable , easy method , price is not an issue .

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#### xenos

##### Full Member level 4
You have a number of posible solutions available, some of them that I can quickly recall:
a) make the power supply levels you require (resistors, capacitors) and use
a1) optocoupler (eg 4N26) to control fet gate by mC
a2) pnp bipolar transistor to control it
b) use PVI5050 (isolated voltage generator for fet driving) - no other power source required
c) use an isolated dc-dc converter 1W module: RY-2405 or equivalent

Electro nS

### Electro nS

Points: 2

#### FvM

##### Super Moderator
Staff member
What's your problem with the circuit, it's rather straightforward.
1. A self-oscillating flyback converter
2. A transistor to speed up switch-off operation

The diode in parallel to the transistor is a zener (e.g. 12 V) that keeps the gate voltage constant over input voltage variations.

From the options listed by xenos I would prefer the photovoltaic opto coupler if switching speed and isolation are sufficient. A recent type with good switching behaviour is e.g. Vishay VOM1271.

Electro nS

### Electro nS

Points: 2

thanks again .

#### FvM

##### Super Moderator
Staff member
Switching speed and output voltage (8v for vom1271 , i have tested it , around 20ms to turn on ) is not enough
Sounds like a really big FET. Specified ton with 200 pF gate capacitance is 50 µs. But I agree that the solution may be insufficient for some applications.

Do you intend to apply the input signal directly to the DC/DC converter? I fear, the response time with an industry standard DC/DC converter could be larger than 20 ms. If response time matters, you'll go for a permanent driver supply through DC/DC and an optocoupler for the control signal.

assembling a circuit is much more expensive and time consuming
Yes, unfortunately.

Electro nS

### Electro nS

Points: 2

#### Electro nS

##### Full Member level 6
Sounds like a really big FET. Specified ton with 200 pF gate capacitance is 50 µs. But I agree that the solution may be insufficient for some applications.

Do you intend to apply the input signal directly to the DC/DC converter? I fear, the response time with an industry standard DC/DC converter could be larger than 20 ms. If response time matters, you'll go for a permanent driver supply through DC/DC and an optocoupler for the control signal.

Yes, unfortunately.

It is indeed a big fet with approximatly 1000nC total charge and Input Capacitance of 30,000pF !
my calculation for gate drive requirment : i=dq/dt ; for q=1000nC , ton=1ms , i=1mA ; and according to i=C*dv/dt
i= 30000*10^-12*(12v/1ms)=3.6*10^-1mA . my humble calculations i found that 1mA is enough to turn on this fet in 1ms

The response time is the delay after the output becomes stable ??, does this affect the rise time of the mosfet ?
my application is not that sensitive , it is ok if the open/close switch order is delayed . but the mosfet should open quickly , because if fear if the mosfet is taking 5ms to 20ms to turn on; during this time it is NOT in the saturation region and it is half open so i fear it will get heated up quickly and destroyed by the load current (load is a motor 36v 100A pump) ! please correct me if i am wrong . the switching ON/OFF will occur every 10seconds , so it is very very very slow .
i posted a schematic please take a look and comment

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#### Warpspeed

. If response time matters, you'll go for a permanent driver supply through DC/DC and an optocoupler for the control signal.
This is way to do it.

Electro nS

### Electro nS

Points: 2

#### schmitt trigger

The problem may be that classical electronic circuits like the simple one-transistor blocking oscillator aren't taught any more today, people apparently can't think of making an electronic circuit without an "IC", e.g. a 555. https://en.wikipedia.org/wiki/Blocking_oscillator

I advise people to build simple "joule thief" circuit for them to get acquainted with this basic -yet very effective- topology.

EVERYONE is amazed at the performance of such a simple circuit. They can't believe one doesn't require the latest digital IC to build something useful.

For a while I've been toying with the idea to suggest to this website's administrators to change the name of the "Tubes and Retro" forum to "Tubes, Retro and Discrete Component". I am sure that forum would get many more entries.

Electro nS

### Electro nS

Points: 2

#### Electro nS

##### Full Member level 6
This is way to do it.

is this reply for me ? , you mean according to FVM instructions my circuits is correct ?

thanks

#### Warpspeed

Sorry I was not more clear.

I agree with FvM, use a dc/dc converter that runs all the time to generate a constant isolated supply, then use an opto coupler to switch your mosfet gate.
It will allow very fast clean switching without any problems.

Electro nS

### Electro nS

Points: 2

#### Electro nS

##### Full Member level 6
for others who might be interested in the same subject i found an article that summaries a lot of what have been mentioned here : **broken link removed**

thanks all for help , i got it to work perfectly now , will mark as solved

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