slope compensation

[Heindorf]

Hi all,
somehow I can't get the point of slope compensation.We're currently
working on a power supply, full bridge phase shifted voltage mode with
the LTC3722-2.Now there is an imbalance in the current distribution of
the 2 legs.To tackle the problem current mode control is recommended.The
IC supports both modes.It is also shown that to avoid instability a
slope compensation should be added for duty cycles>50%.Does that mean
that my available output current decreases also to 50%?
Let's take our example. At full power we get at 99% duty cycle approx.
10A Bridge current corresponding to 4.5V Error Amplifier output.If I
insert now a 0.45 Ohm resistor I get 4.5V at 10A.If I add a slope
compensation would I decrease the duty cycle and hence the bridge
current? Or do I need another transformer to get the same power with 50%
duty cycle? I have here some conceptual difficulties I guess.
Rolf

 

[Heindorf]

Hi Mook,
I've already read some app. notes in this respect and wouldn't have posted if
there were
not so many important questions unanswered, at least for me.
Rolf

Mook Johnson schrieb:

search the TI website under power management. There are plenty of
explanations in the application notes on exactly what slope compensation is
and how to implement it.

"Heindorf" <heindorf@aeras.de> wrote in message
news:41ECF089.35340958@aeras.de...
Hi all,
somehow I can't get the point of slope compensation.We're currently
working on a power supply, full bridge phase shifted voltage mode with
the LTC3722-2.Now there is an imbalance in the current distribution of
the 2 legs.To tackle the problem current mode control is recommended.The
IC supports both modes.It is also shown that to avoid instability a
slope compensation should be added for duty cycles>50%.Does that mean
that my available output current decreases also to 50%?
Let's take our example. At full power we get at 99% duty cycle approx.
10A Bridge current corresponding to 4.5V Error Amplifier output.If I
insert now a 0.45 Ohm resistor I get 4.5V at 10A.If I add a slope
compensation would I decrease the duty cycle and hence the bridge
current? Or do I need another transformer to get the same power with 50%
duty cycle? I have here some conceptual difficulties I guess.
Rolf

 

[Genome]

"Heindorf" <heindorf@aeras.de> wrote in message
news:41ECF089.35340958@aeras.de...

Hi all,
somehow I can't get the point of slope compensation.We're currently
working on a power supply, full bridge phase shifted voltage mode with
the LTC3722-2.Now there is an imbalance in the current distribution of
the 2 legs.To tackle the problem current mode control is recommended.The
IC supports both modes.It is also shown that to avoid instability a
slope compensation should be added for duty cycles>50%.Does that mean
that my available output current decreases also to 50%?
Let's take our example. At full power we get at 99% duty cycle approx.
10A Bridge current corresponding to 4.5V Error Amplifier output.If I
insert now a 0.45 Ohm resistor I get 4.5V at 10A.If I add a slope
compensation would I decrease the duty cycle and hence the bridge
current? Or do I need another transformer to get the same power with 50%
duty cycle? I have here some conceptual difficulties I guess.
Rolf

Hope you're on a fixed width font....

"Hi all, somehow I can't get the point of slope compensation."

Me neither......but, if you are using 'peak' current mode control then your
supply can get this answer.

__________________________________________ error signal
/| /| /| /| /| /|
| | / | | | / | | | / |
| | / | | | / | | | / |
| |___| |__| |____| |__| |____| |__ primary current sense signal

|_____|______|______|______|______|______| clock signal

/| /| /| /| /|
/ | / | / | / | / |
/ | / | / | / | / | /
/ | / | / | / | / | /
/ | / | / | / | / | /
/ |/ |/ |/ |/ |/ ramp signal




When you were expecting this...




__________________________________________ error signal
/| /| /| /| /| /|
/ | / | / | / | / | / |
| | | | | | | | | | | |
| |__| |___| |___| |___| |___| |__ primary current sense signal

|_____|______|______|______|______|______| clock signal

/| /| /| /| /|
/ | / | / | / | / |
/ | / | / | / | / | /
/ | / | / | / | / | /
/ | / | / | / | / | /
/ |/ |/ |/ |/ |/ ramp signal


With peak current limiting the answers are the same.

Unfortunately your supply is guaranteed to lock itself
into the first one, AKA subharmonic oscillation.

I can't explain why it should, I'm thick as a brick.

You avoid this by adding slope compensation. You add
part of the ramp signal to the current sense signal.

Something late has more ramp and gets terminated earlier.
Something early has less ramp and gets terminated later.

So they sort of get equalised.

The magic sum is... can't remember... but if you add half
the equivalent sensed inductor downslope then your supply
will be dealing with average output current.

Which might be quite good.

The other magic sum is you can do something else that makes
your supply invarient to the input volts or somesuch.


YES it does fuck about with your current limit if your
current limit depends on the peak sensed current.

That means you get to solve the 'Three Resistors From Hell'
problem and worry about what happens when the supply goes
into current limit.

DNA

 

[legg]

On Wed, 19 Jan 2005 11:58:30 +0100, Heindorf <heindorf@aeras.de>
wrote:

Hello Genome,
thank you for your efforts and also all the other answers.We have a fixed
frequency configuration.I must think about all ideas and remarks-quite a lot of
stuff so far.But one main concern of me hasn't been addressed, I suppose.In a
bridge topology with a power transformer I design the transformer for 100% DF
(voltage mode).If I now slope compensate the circuit to below 50% in current
mode, do I need another transformer or is this compensation never applied to
(phase shifted) bridge configurations?
There is a lot unclear to me.

The simple addition of slope compensation would not affect your
controller's duty cycle limits.

This limit is unlikely to be 100%, using most of the available control
devices, particularly at full load. This is due to arbitrary
imposition of a minimum dead-time, and the inability of power switches
to reverse large currents in the primary, without delays.

RL

 

[legg]

On Wed, 19 Jan 2005 14:12:46 +0100, Heindorf <heindorf@aeras.de>
wrote:

I add slope compensation, get a DF below 50% and don't affect the duty
cycle limit?Could you elaborate,please?

The addition of slope compensation will not affect the duty cycle (~
Duty Factor), nor will it affect the duty cycle limit of the
controller.

Your reference to 50% duty may be confusion over the noted fact that,
for some of the affected topologies, stability is achievable without
compensation, below 50% duty. This is the only reason a writer would
mention slope compensation and 50% duty cycle on the same page.

Duty cycle is primarily determined by input/output voltage ratios,
(reflected through the isolating transformer turns ratio, as may be
the case).

RL

 

[Heindorf]

Hello all,
thanks for all contributions. I think it will help me a lot.
Regards
Rolf

optoeng@pioneernet.net schrieb:

1. In full-bridge converters operating in Continuous Current Mode (CCM,
where the smoothing inductor current never goes to zero) with
approximately constant DC voltage input, average power output and
average DF are not linearly related. The instantaneous value of Duty
Factor varies to maintain regulation, but the average value of DF
depends mainly on input to output voltage ratio and turns ratio. As
the load draws more power from the secondary, the DC current in the
inductor rises along with load current. Of course, the ripple current
in the inductor (which is the primary ripple reflected through the
turns ratio) increases in amplitude as load current rises, but this
ripple is typically only about 10% to 30% of the DC current. In other
words, double the load doesn't require double the DF. (To put it yet
another way, the transformer in a full-bridge converter acts like a
power transformer.)

2. Full-bridge converters drive the primary with alternating polarity
pulses. In order to prevent core saturation, these pulses must have a
net volt-seconds integral (sum) of zero. If this doesn't obtain, even
if imbalances are small, the core will eventually saturate (flux
walking). Many methods have been developed to prevent this, most
notably: air gaps in the core and capacitive coupling of the primary.
In theory, current mode feedback control also prevents the flux walking
phenomenon. In practice, it is difficult to achieve balance using
feedback over the great variety of operating conditions, including
power-up, power-down, overloads, etc.

3. Using peak current for feedback has its advantages, but one
disadvantage is the susceptibility of the system to subharmonic
oscillations. Much good literature is available on this subject. I
suggest starting at:

http://www.ridleyengineering.com

The TI/Unitrode Switchmode seminar series is also easy to find on the
web and full of great information. Here's a paper you should memorize:

http://www.powerdesigners.com/InfoWeb/design_center/Appnotes_Archive/u111.pdf

4. If there is any tendence toward subharmonic oscillation in a
full-bridge system, then you are almost guaranteed to have flux walking
problems. This is because all the even subharmonics have DC offsets.
For example, the first subharmonic has different amplitudes for every
other controller cycle. For a full-bridge, this means that the
positive half-cycles are offset from the negative half-cycles. Yikes!
For this reason, until I have a stable control loop, I ALWAYS
capacitively couple the primary of any full-bridge converter. IFF I'm
able to achieve the phase margins that I need, I SOMETIMES return to DC
coupling.

Paul Mathews

 

[Mook Johnson]

search the TI website under power management. There are plenty of
explanations in the application notes on exactly what slope compensation is
and how to implement it.


"Heindorf" <heindorf@aeras.de> wrote in message
news:41ECF089.35340958@aeras.de...

Hi all,
somehow I can't get the point of slope compensation.We're currently
working on a power supply, full bridge phase shifted voltage mode with
the LTC3722-2.Now there is an imbalance in the current distribution of
the 2 legs.To tackle the problem current mode control is recommended.The
IC supports both modes.It is also shown that to avoid instability a
slope compensation should be added for duty cycles>50%.Does that mean
that my available output current decreases also to 50%?
Let's take our example. At full power we get at 99% duty cycle approx.
10A Bridge current corresponding to 4.5V Error Amplifier output.If I
insert now a 0.45 Ohm resistor I get 4.5V at 10A.If I add a slope
compensation would I decrease the duty cycle and hence the bridge
current? Or do I need another transformer to get the same power with 50%
duty cycle? I have here some conceptual difficulties I guess.
Rolf

 

[Heindorf]

Hello Genome,
thank you for your efforts and also all the other answers.We have a fixed
frequency configuration.I must think about all ideas and remarks-quite a lot of
stuff so far.But one main concern of me hasn't been addressed, I suppose.In a
bridge topology with a power transformer I design the transformer for 100% DF
(voltage mode).If I now slope compensate the circuit to below 50% in current
mode, do I need another transformer or is this compensation never applied to
(phase shifted) bridge configurations?
There is a lot unclear to me.
Rolf

Genome schrieb:

"Heindorf" <heindorf@aeras.de> wrote in message
news:41ECF089.35340958@aeras.de...
Hi all,
somehow I can't get the point of slope compensation.We're currently
working on a power supply, full bridge phase shifted voltage mode with
the LTC3722-2.Now there is an imbalance in the current distribution of
the 2 legs.To tackle the problem current mode control is recommended.The
IC supports both modes.It is also shown that to avoid instability a
slope compensation should be added for duty cycles>50%.Does that mean
that my available output current decreases also to 50%?
Let's take our example. At full power we get at 99% duty cycle approx.
10A Bridge current corresponding to 4.5V Error Amplifier output.If I
insert now a 0.45 Ohm resistor I get 4.5V at 10A.If I add a slope
compensation would I decrease the duty cycle and hence the bridge
current? Or do I need another transformer to get the same power with 50%
duty cycle? I have here some conceptual difficulties I guess.
Rolf


Hope you're on a fixed width font....

"Hi all, somehow I can't get the point of slope compensation."

Me neither......but, if you are using 'peak' current mode control then your
supply can get this answer.

__________________________________________ error signal
/| /| /| /| /| /|
| | / | | | / | | | / |
| | / | | | / | | | / |
| |___| |__| |____| |__| |____| |__ primary current sense signal

|_____|______|______|______|______|______| clock signal

/| /| /| /| /|
/ | / | / | / | / |
/ | / | / | / | / | /
/ | / | / | / | / | /
/ | / | / | / | / | /
/ |/ |/ |/ |/ |/ ramp signal

When you were expecting this...

__________________________________________ error signal
/| /| /| /| /| /|
/ | / | / | / | / | / |
| | | | | | | | | | | |
| |__| |___| |___| |___| |___| |__ primary current sense signal

|_____|______|______|______|______|______| clock signal

/| /| /| /| /|
/ | / | / | / | / |
/ | / | / | / | / | /
/ | / | / | / | / | /
/ | / | / | / | / | /
/ |/ |/ |/ |/ |/ ramp signal

With peak current limiting the answers are the same.

Unfortunately your supply is guaranteed to lock itself
into the first one, AKA subharmonic oscillation.

I can't explain why it should, I'm thick as a brick.

You avoid this by adding slope compensation. You add
part of the ramp signal to the current sense signal.

Something late has more ramp and gets terminated earlier.
Something early has less ramp and gets terminated later.

So they sort of get equalised.

The magic sum is... can't remember... but if you add half
the equivalent sensed inductor downslope then your supply
will be dealing with average output current.

Which might be quite good.

The other magic sum is you can do something else that makes
your supply invarient to the input volts or somesuch.

YES it does fuck about with your current limit if your
current limit depends on the peak sensed current.

That means you get to solve the 'Three Resistors From Hell'
problem and worry about what happens when the supply goes
into current limit.

DNA