Junkbox DC-DC Converter

[Randy Day]

I worked up the circuit below to create an isolated DC
supply, and it works (sort of). It's a 555 astable running
at ~500KHz into a voltage doubler.

I get an unloaded voltage of 7v, dropping to 1.5v under
load. My problem is that I'd like to get >5v at 20ma
continuous out of it. Any suggestions on how to improve the
current output (assuming it's possible)?

Failing that, can anyone point me to a simple 5v-5v
DC converter circuit w/isolation?

+5v
|-------+-+
.-. | |
| | .----------.
1k | | | 4 8 | 100nf
'-' | | ||
+----|7 3|-||-+--->|--+------+ X---|
.-. | | || | | | |
| | | LM555 | | |10uf | |
10k | | | | | --- | |
'-' | | - --- | |
|--+-|2 | ^ | |10uf |
100 --- +-|6 5 1 | | | --- .-.
pf --- '----------' || | | --- | |
| | +---||-|-------+ | | |Rload
GND --- | || | |10uf | '-'
--- | 100nf| --- | |
| GND | --- | |
GND | | | |
|-------+------+ X---|

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-
chat.de)

 

[Tim Wescott]

Randy Day wrote:

I worked up the circuit below to create an isolated DC
supply, and it works (sort of). It's a 555 astable running
at ~500KHz into a voltage doubler.

I get an unloaded voltage of 7v, dropping to 1.5v under
load. My problem is that I'd like to get >5v at 20ma
continuous out of it. Any suggestions on how to improve the
current output (assuming it's possible)?

Failing that, can anyone point me to a simple 5v-5v
DC converter circuit w/isolation?

+5v
|-------+-+
.-. | |
| | .----------.
1k | | | 4 8 | 100nf
'-' | | ||
+----|7 3|-||-+--->|--+------+ X---|
.-. | | || | | | |
| | | LM555 | | |10uf | |
10k | | | | | --- | |
'-' | | - --- | |
|--+-|2 | ^ | |10uf |
100 --- +-|6 5 1 | | | --- .-.
pf --- '----------' || | | --- | |
| | +---||-|-------+ | | |Rload
GND --- | || | |10uf | '-'
--- | 100nf| --- | |
| GND | --- | |
GND | | | |
|-------+------+ X---|

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-
chat.de)

Do you have an O-scope? If so, check the voltage on pin 3 under load --
I wouldn't be surprised if you found not able to drive the thing.

Also, what diodes are you using? Something like a 1N4148 should be fast
enough (although I don't know if it's rated to 20mA); something like a
1N4001 would be _way_ too slow.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[IanM]

Tim Wescott wrote:

Randy Day wrote:

I worked up the circuit below to create an isolated DC supply, and it
works (sort of). It's a 555 astable running at ~500KHz into a voltage
doubler.

I get an unloaded voltage of 7v, dropping to 1.5v under load. My
problem is that I'd like to get >5v at 20ma continuous out of it. Any
suggestions on how to improve the current output (assuming it's
possible)?
Failing that, can anyone point me to a simple 5v-5v
DC converter circuit w/isolation?

+5v
|-------+-+
.-. | |
| | .----------.
1k | | | 4 8 | 100nf
'-' | | ||
+----|7 3|-||-+--->|--+------+ X---|
.-. | | || | | | |
| | | LM555 | | |10uf | |
10k | | | | | --- | |
'-' | | - --- | |
|--+-|2 | ^ | |10uf |
100 --- +-|6 5 1 | | | --- .-.
pf --- '----------' || | | --- | |
| | +---||-|-------+ | | |Rload
GND --- | || | |10uf | '-'
--- | 100nf| --- | |
| GND | --- | |
GND | | | |
|-------+------+ X---|

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-
chat.de)


Do you have an O-scope? If so, check the voltage on pin 3 under load --
I wouldn't be surprised if you found not able to drive the thing.

Also, what diodes are you using? Something like a 1N4148 should be fast
enough (although I don't know if it's rated to 20mA); something like a
1N4001 would be _way_ too slow.

With that circuit configuration, there is NO WAY to get more than the

output swing of the 555 minus two diode drops at any real load current
(the 7v observed at no load is almost certainly due to ringing on the
555 O/P).

With a 5 V supply, I'd be amazed if it could even manage 4V into a 10K load.

Also the two series 10uF caps are not needed at all. The 100nF to ground
might as well go to the negative side of the load instead of their
junction and if you need more reservoir capacitance, increase the value
of the remaining 10uF cap.
It would also be pretty much essential to have a decoupling capacitor
across pins 1 & 8 of the 555. The 555 is operating at somewhere close
to a 45% duty cycle so to get 20mA out it must sink & source over 45mA.
At that level of output current, from the curves in the data sheet,
your output swing is from about Vdd -1.5V to Gnd +1V (curves only valid
for Vss>5v.) Thus you are loosing another 2.5V. This is consistent
with your 1.5V measured under load.

Converting it to a voltage doubler will loose you two more diode drops
and halve the load current so its fairly obvious that you cant make a
555 based charge pump converter do what you want.

If you can supply the 555 with a minimum of 9V you might be fairly happy
with the output.

Does the 5V out need to be regulated?
Why do you need the isolation?
What other constraints are there?

Might be easiest to tell us what its for.

 

[John Popelish]

Randy Day wrote:

I worked up the circuit below to create an isolated DC
supply, and it works (sort of). It's a 555 astable running
at ~500KHz into a voltage doubler.

I get an unloaded voltage of 7v, dropping to 1.5v under
load. My problem is that I'd like to get >5v at 20ma
continuous out of it. Any suggestions on how to improve the
current output (assuming it's possible)?

Failing that, can anyone point me to a simple 5v-5v
DC converter circuit w/isolation?

Ive made some changes to your schematic that might help.
The two diodes should be 1N914, 1N4148, 1N4448, etc.

+5v
|-------+-+
.-. | |
| | .----------.
1k | | | 4 8 | 100nf
'-' | | ||
+----|7 3|-||-+--->|--------+ X---|
.-. | | || | | |
| | | LM555 | | | |
10k | | | | | | |
'-' | | - | |
|--+-|2 | ^ |10uf |
100 --- +-|6 5 1 | | --- .-.
pf --- '----------' +5v --- | |
| | | | |Rload
GND --- | '-'
--- | |
| | |
GND | |
+ X---|
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-
chat.de) (and edited by hand)

--
Regards,

John Popelish

 

[Randy Day]

In article <gd0nkk$46t$1@energise.enta.net>,
Invalid@totally.invalid says...

[snip]

If you can supply the 555 with a minimum of 9V you might be fairly happy
with the output.

Not an option, I'm afraid...

Does the 5V out need to be regulated?

Yes.

Why do you need the isolation?

There's a possibility (low, but nonzero) of a ground fault
or other electrical malfunction putting line AC onto the
load side ground line. This would pass back through cables
and equipment that really aren't designed for it.

What other constraints are there?

Might be easiest to tell us what its for.

A USB-powered device. A usb-capable PIC communicating via
opto-isolators with the load side. This was my first attempt
at getting power across the divide.

I was hoping to avoid a bulky isolation xfmr, but I can bite
the bullet if I have to.

 

[IanM]

Randy Day wrote:

In article <gd0nkk$46t$1@energise.enta.net>,
Invalid@totally.invalid says...

[snip]

Why do you need the isolation?

There's a possibility (low, but nonzero) of a ground fault
or other electrical malfunction putting line AC onto the
load side ground line. This would pass back through cables
and equipment that really aren't designed for it.

What other constraints are there?

Might be easiest to tell us what its for.


A USB-powered device. A usb-capable PIC communicating via
opto-isolators with the load side. This was my first attempt
at getting power across the divide.

I was hoping to avoid a bulky isolation xfmr, but I can bite
the bullet if I have to.
I dont see any way you can win with your 555 based circuit then. The

capacitive coupling would also not have been a great idea if it is
important to keep mains spikes and other transients out of the USB side

Is there any way you could use an AC line derived supply on the load
side? You dont actually need a transformer then but could use a
capacitive dropper feeding a rectifier and shunt regulator. If you are
doing any power control on the load side, a line derived supply is
pretty much essential so that you can lock out the drive to the output
stage if the USB side looses power. You'll probably also want to
optoisolate a 'power good' signal to the PIC to let you manage startup
and shutdown cleanly.

OTOH if its just sensors on the load side, a DC-DC convertor solution
off the USB power makes sense. I doubt I'd roll my own though.
Unless you are producing a large number of these devices, buying in an
isolated switched mode 5V converter module such as:

<http://uk.rs-online.com/web/5105431.html>

looks rather attractive. Reasonable price and you aren't going to get
much more compact.

Beware of limited current availablility on the USB bus. If the device
hasn't enumerated yet or someone has plugged it into a bus powered hub,
you may have to keep most of your device powered down, indicating an
error code if you have to.

For prototyping purposes, a small 'wall wart' and a 5V regulator should
be findable in your junk box while you are waiting for yor converter
module order to come in.

 

[ehsjr]

Randy Day wrote:

I worked up the circuit below to create an isolated DC
supply, and it works (sort of). It's a 555 astable running
at ~500KHz into a voltage doubler.

I get an unloaded voltage of 7v, dropping to 1.5v under
load. My problem is that I'd like to get >5v at 20ma
continuous out of it. Any suggestions on how to improve the
current output (assuming it's possible)?

Failing that, can anyone point me to a simple 5v-5v
DC converter circuit w/isolation?

+5v
|-------+-+
.-. | |
| | .----------.
1k | | | 4 8 | 100nf
'-' | | ||
+----|7 3|-||-+--->|--+------+ X---|
.-. | | || | | | |
| | | LM555 | | |10uf | |
10k | | | | | --- | |
'-' | | - --- | |
|--+-|2 | ^ | |10uf |
100 --- +-|6 5 1 | | | --- .-.
pf --- '----------' || | | --- | |
| | +---||-|-------+ | | |Rload
GND --- | || | |10uf | '-'
--- | 100nf| --- | |
| GND | --- | |
GND | | | |
|-------+------+ X---|

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-
chat.de)

This might work for you, depending on what you have in
the junkbox:


To bridge rectifier
| |
[Xformer]
| | |
+-------------+---+ + +---+-----------------+
| | | | |
| +----- | ---- | ---+ |
| | | | | |
| [.1uf] | | | |
| | | | | |
| +----- | -----+ | |
\c | [R] | c/
|---[10K]---+ | +---[10K]---|
/e +5 Vcc e\
| |
| |
+--------------------+------------------------+
|
Gnd

Limit the collector current with R.

Ed