pwm chip

[kell]

I would like to wire up a current mode pwm chip to drive an inductor to
a specific current each cycle. Take the UC3842/3 chip for example:

http://rocky.digikey.com/WebLib/On-Semi/Web%20Data/UC2842B_43B,%20UC3842B_43B,%20NCV3843BV.pdf

This data sheet has a formula on page 8 (under "Operating Description")
for peak inductor current in terms of Error Amp Output. I want to
calculate Rs that will give me a certain Ipk. In the formula, what
value do I use for the V(Pin1) term. Do I have to connect pins 1 and 2
with a resistor as in figure 32 on page 14, which would make V(Pin1)
2.5 volts?

 

[Winfield Hill]

kell wrote...

I would like to wire up a current mode pwm chip to drive an inductor to
a specific current each cycle. Take the UC3842/3 chip for example:

http://rocky.digikey.com...

DigiKey only has rev 3 of the datasheet, ON Semi is up to rev 6.
http://www.onsemi.com/pub/Collateral/UC3842B.PDF

Many other manufacturers have uc3843 info, TI (who bought Unitrode,
who introduced the chip 15 years ago), Fairchild, ST, Microsemi,
Linfinity, Bay Linear, Unisonic, etc... The TI UCC38Cxx ICs are
advanced low-power BiCMOS versions of the original Unitrode parts.

And check out TI's slua143 U-100A Unitrode 14-page app note.

This data sheet has a formula on page 8 (under "Operating Description")
for peak inductor current in terms of Error Amp Output. I want to
calculate Rs that will give me a certain Ipk. In the formula, what
value do I use for the V(Pin1) term. Do I have to connect pins 1 and
2 with a resistor as in figure 32 on page 14, which would make V(Pin1)
2.5 volts?

Normally the pin 1 voltage results from an error-correction feedback
loop, so the relevant component in figure 32 is the integrating cap.
But you can wire the controller for a fixed current level by using
the opamp, or by otherwise forcing a voltage on pin 1. Ground pin 2
to make pin 1 high, which is a roughly 1mA current source, and over-
ride that with your setpoint voltage. Keep in mind the approximate
nature of the page 8 formula, Ipk = (V1 - 1.4) / 3 Rs, where the 1.4
and 3 factors are uncertain, and a further offset error isn't shown.


--
Thanks,
- Win

 

[kell]

Putting the question another way -- I still want to use feedback how
would I know the peak inductor current at such times as low feedback
voltage on pin 2 causes it to drive full-out. I can't force pin 1
without defeating the feedback.
I will have to look at some of those other data sheets for an
explanation. Perhaps it has to do with the integrating cap you
mentioned.

 

[Winfield Hill]

kell wrote...

Putting the question another way -- I still want to use feedback how
would I know the peak inductor current at such times as low feedback
voltage on pin 2 causes it to drive full-out. I can't force pin 1
without defeating the feedback.
I will have to look at some of those other data sheets for an
explanation. Perhaps it has to do with the integrating cap you
mentioned.

What feedback, and for what purpose? Explain yourself, man!


--
Thanks,
- Win

 

[kell]

This is a circuit that charges a car battery using pulses from an
inductor. The idea is to feed back a divided-down battery voltage to
pin 2 for regulation when the battery approaches full charge and its
voltage rises near the selected set point.

But when the battery is low the pwm chip drives the inductor to some
peak current. This being the highest current the inductor will see, I
am trying to determine its value. I want to drive the inductor to a
peak current that is high but not so high as to saturate it. Let's say
for my selected inductor that current is 2 amps. So now how do I
determine the value for the sense resistor that will cause the chip to
drive the inductor to two amps when the battery is low and the chip is
driving full out.
Hope I gave a comprehensible explanation. Kell

 

[Fabio G.]

"kell" <kellrobinson@billburg.com> ha scritto:

But when the battery is low the pwm chip drives the inductor to some
peak current.

Batteries are charged with the mean current value, not with the peak
current value.
In discontinuos mode (which often appears in battery charging) peak
current and mean current are very different.
You may encounter unaccurate battery charging without a proper
compensation.

Average current mode can be obtained with the slope compensation
technique.

Bye

--
Per rispondermi via email sostituisci il risultato
dell'operazione (in lettere) dall'indirizzo

 

[kell]

I want to find the peak inductor current. This bears on the selection
of the inductor, not on the average current going to the battery.
I asked a straight question about the operation of the chip with
voltage feedback. So think of it as a boost converter circuit, with a
voltage divider from the output going to pin 2. How does the sense
resistor value relate to the peak inductor current when the output
voltage is low (low signal to pin 2). If the chip does not operate
this way tell me. I can take it.

 

[kell]

By the way, Fabio et al.
That 2 amps I mentioned is not the charging current going into the
battery. You jumped to the conclusion that I really know so little as
to equate peak inductor current with average current. I don't even
have to know the average current; I can just regulate the battery's
voltage, kapeesh? That's why talk about VOLTAGE FEEDBACK. When I put
the battery on to charge, I can set the voltage to 14.5 volts or so.
Then when I think it has taken on most of the bulk charge, I can set
the voltage back to about 13.5 volts. I don't want to consider
monitoring charging current right now.
So.

 

[Terry Given]

John Popelish wrote:

kell wrote:

By the way, Fabio et al.
That 2 amps I mentioned is not the charging current going into the
battery. You jumped to the conclusion that I really know so little as
to equate peak inductor current with average current. I don't even
have to know the average current; I can just regulate the battery's
voltage, kapeesh? That's why talk about VOLTAGE FEEDBACK. When I put
the battery on to charge, I can set the voltage to 14.5 volts or so.
Then when I think it has taken on most of the bulk charge, I can set
the voltage back to about 13.5 volts. I don't want to consider
monitoring charging current right now.
So.


Page 8 of the data sheet covers your question. I think.
Here is a clip from that page:

Current Sense Comparator and PWM Latch
The UC3842B, UC3843B operate as a current mode
controller, whereby output switch conduction is initiated by
the oscillator and terminated when the peak inductor current
reaches the threshold level established by the Error
Amplifier Output/Compensation (Pin 1). Thus the error
signal controls the peak inductor current on a
cycle–by–cycle basis. The Current Sense Comparator PWM
Latch configuration used ensures that only a single pulse
appears at the Output during any given oscillator cycle. The
inductor current is converted to a voltage by inserting the
ground–referenced sense resistor RS in series with the
source of output switch Q1. This voltage is monitored by the
Current Sense Input (Pin 3) and compared to a level derived
from the Error Amp Output. The peak inductor current under
normal operating conditions is controlled by the voltage at
pin 1 where:
Ipk = (V(Pin 1) – 1.4 V)/(3 RS)

Abnormal operating conditions occur when the power
supply output is overloaded or if output voltage sensing is
lost. Under these conditions, the Current Sense Comparator
threshold will be internally clamped to 1.0 V. Therefore the
maximum peak switch current is:
Ipk(max) =1.0 V/RS

Before your battery reaches the voltage set point, you are effectively
in overload condition, so the second formula applies to the current
limit.
RS is the current sense resistor in the source lead of the external
mosfet.

Once the voltage approaches the setpoint, the voltage error amplifier
lowers the current limit and the first formula takes over.
The voltage on pin 1 (which is the output of the voltage error
amplifier) depends on the gain and compensation network you connect
between pin 1 and pin 2 and the voltage divider network between pin 2
and the output voltage.

bingo.

IOW, your voltage feedback circuitry saturates the EA, up until Vbatt
reaches some suitable value (perhaps 12V, whatever). During that time,
thye 1V zener on the internal CS comparator input sets the reference
voltage to 1V (ish - read the datasheet for the tolerance, it is given).
So Ipeak = 1V/Rs

you dont *have* to have an integrator for the EA BTW - fixed gain is
also possible, as is any other arbitrarily complex feedback network.
fixed gain might be just what you want....

Cheers
Terry

 

[John Popelish]

kell wrote:

By the way, Fabio et al.
That 2 amps I mentioned is not the charging current going into the
battery. You jumped to the conclusion that I really know so little as
to equate peak inductor current with average current. I don't even
have to know the average current; I can just regulate the battery's
voltage, kapeesh? That's why talk about VOLTAGE FEEDBACK. When I put
the battery on to charge, I can set the voltage to 14.5 volts or so.
Then when I think it has taken on most of the bulk charge, I can set
the voltage back to about 13.5 volts. I don't want to consider
monitoring charging current right now.
So.

Page 8 of the data sheet covers your question. I think.
Here is a clip from that page:

Current Sense Comparator and PWM Latch
The UC3842B, UC3843B operate as a current mode
controller, whereby output switch conduction is initiated by
the oscillator and terminated when the peak inductor current
reaches the threshold level established by the Error
Amplifier Output/Compensation (Pin 1). Thus the error
signal controls the peak inductor current on a
cycle–by–cycle basis. The Current Sense Comparator PWM
Latch configuration used ensures that only a single pulse
appears at the Output during any given oscillator cycle. The
inductor current is converted to a voltage by inserting the
ground–referenced sense resistor RS in series with the
source of output switch Q1. This voltage is monitored by the
Current Sense Input (Pin 3) and compared to a level derived
from the Error Amp Output. The peak inductor current under
normal operating conditions is controlled by the voltage at
pin 1 where:
Ipk = (V(Pin 1) – 1.4 V)/(3 RS)

Abnormal operating conditions occur when the power
supply output is overloaded or if output voltage sensing is
lost. Under these conditions, the Current Sense Comparator
threshold will be internally clamped to 1.0 V. Therefore the
maximum peak switch current is:
Ipk(max) =1.0 V/RS

Before your battery reaches the voltage set point, you are effectively
in overload condition, so the second formula applies to the current
limit.
RS is the current sense resistor in the source lead of the external
mosfet.

Once the voltage approaches the setpoint, the voltage error amplifier
lowers the current limit and the first formula takes over.
The voltage on pin 1 (which is the output of the voltage error
amplifier) depends on the gain and compensation network you connect
between pin 1 and pin 2 and the voltage divider network between pin 2
and the output voltage.

--
John Popelish