Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

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

Status
Not open for further replies.

Electro nS

Full Member level 6
Joined
Mar 9, 2013
Messages
323
Helped
49
Reputation
98
Reaction score
47
Trophy points
1,308
Activity points
3,786
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 .
what is your opinion

thanks in advance for help
 

Attachments

Last edited:

xenos

Full Member level 4
Joined
May 9, 2015
Messages
212
Helped
82
Reputation
164
Reaction score
81
Trophy points
28
Location
127.0.0.1
Activity points
1,182
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
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,436
Helped
14,034
Reputation
28,321
Reaction score
12,684
Trophy points
1,393
Location
Bochum, Germany
Activity points
275,912
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

Full Member level 6
Joined
Mar 9, 2013
Messages
323
Helped
49
Reputation
98
Reaction score
47
Trophy points
1,308
Activity points
3,786
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

.
it is that straight forward to you my friend ? :p , i have been shaking my head around it for several hours!! the thing that i cannot under stand is the transformer , usually there is primary and secondary , how is the third coil connected is puzzling me , can u provide more information or an application note for more clarification. or a datasheet for similar transformer

thank u very much
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,436
Helped
14,034
Reputation
28,321
Reaction score
12,684
Trophy points
1,393
Location
Bochum, Germany
Activity points
275,912
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. http://en.wikipedia.org/wiki/Blocking_oscillator
 

Electro nS

Full Member level 6
Joined
Mar 9, 2013
Messages
323
Helped
49
Reputation
98
Reaction score
47
Trophy points
1,308
Activity points
3,786
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. http://en.wikipedia.org/wiki/Blocking_oscillator
just a question , this type of circuit needs a pulse transformer ? or audio transformer

i really have a hard time choosing , espically such transformer with 3 winding of different number of turns :(
usually available are center tapped example 1:1:3 , and such
please help me with how to design such circuit and find a suitable transformer
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,436
Helped
14,034
Reputation
28,321
Reaction score
12,684
Trophy points
1,393
Location
Bochum, Germany
Activity points
275,912
this type of circuit needs a pulse transformer ? or audio transformer
A transformer is a transformer. Audio, pulse, etc. are just applications involving a number of typical design parameters.

I won't expect a ready made transformer fitting the application, but not impossible. Manufacturers like Coilcraft, Wuerth have a number of small catalog transformers, some may fit. Otherwise there are transformer construction kits helping you to make your own.

I would try determine the parameters of the original transformer like windings ratio, primary and leakage inductance, DC resistance, saturation flux. If not available, redesign the circuit and try to find suitable tarnsformer parameters by calculation and/or empirical tests (or call it trial and error method). You probably end up with different parameters than the original design, no problem as long it's working.
 

Electro nS

Full Member level 6
Joined
Mar 9, 2013
Messages
323
Helped
49
Reputation
98
Reaction score
47
Trophy points
1,308
Activity points
3,786
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.
thank u for your generous reply , if Switching speed and output voltage (8v for vom1271 , i have tested it , around 20ms to turn on ) is not enough . is a small isolated 5 to 12v dc dc converter suffcient for the job? or is there any problems with such choice , since the price is tempting (4$) . (assembling a circuit is much more expensive and time consuming)

thanks again .
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,436
Helped
14,034
Reputation
28,321
Reaction score
12,684
Trophy points
1,393
Location
Bochum, Germany
Activity points
275,912
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

Full Member level 6
Joined
Mar 9, 2013
Messages
323
Helped
49
Reputation
98
Reaction score
47
Trophy points
1,308
Activity points
3,786
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
 
Last edited:

schmitt trigger

Advanced Member level 5
Joined
Apr 17, 2013
Messages
2,677
Helped
847
Reputation
1,698
Reaction score
826
Trophy points
1,393
Activity points
17,964
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. http://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.
 

Warpspeed

Advanced Member level 5
Joined
May 23, 2015
Messages
2,213
Helped
753
Reputation
1,508
Reaction score
731
Trophy points
113
Location
Melbourne, Australia
Activity points
17,766
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.
 
Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top