Section (5)
Power electronics 2
Eng Eng
Mohamed Zaki Mohamed El-madawy
Control scheme of Switching mode power supply
vc = Gc(s)(Vref- vo)
Pulse Width Modulation
PWM is a technique to modify the signal width and number of pulses over the switching time
period.
How can we generate PWM ?
by
1- Modulator signal
called also (modulating or control signal )
may be constant value or sinsuidal
2- carrier signal
is a triangle wave or ramp that controls the switching frequency
Implementation of Pulse Width Modulation
Voltage-Mode (loop) PWM Controller
Two control loops
current-Mode (loop) PWM Controller An inner loop
regulating the current in the switch and inductive
storage element;Il,
An outer loop
which conventionally regulates the output voltage;
Vo with
IL
regard to the internal reference voltage.
the control circuits run at different speeds:
the current control loop reacting pulse by pulse
(faster than the voltage-mode)
The (inductor) current is just subtracted the voltage control loop running much slower to
to the output of the compensation network give an output voltage that is stable over time.
PID controller DC/DC converters Closed loop controller
This type is composed of proportional
(P) term, the integral (I) , and the
derivative (D) control term, to
obtain a desired response.
open loop controller ( on - off control )
Ø The proportional term (Kp) reduces the rise time,
but never eliminate, the steady-state error.
Ø The integral term (Ki) eliminates the steady-
state error, but it may make the transient
response worse.
Ø The derivative term (Kd) increases the stability
of the system, reduces the overshoot, and
improving the transient response.
Ø The previous correlations may not be exactly
accurate: Kp, Ki, and Kd are dependent of each
other. Changing one of these variables can change
the effect of the other two.
The derivative (term) control is
necessary?
· PD is to reduce overshoot or ripple,
· PD controllers are slower than P, but less oscillation, smaller
overshoot and ripple,
· PD controller will not change signal, unless there is change in
output
· If the controller is stable, we can neglect the derivative
term.
Modeling of boost converter
To design a control system, we need a dynamic
AC model of the switching power converters.
Modeling is the representation of physical
circuits by mathematical means
Small Signal Model
Ø There are two well-known variants of ac modeling method:
•State-space averaging, (by using the modeling equations)
•Circuit averaging, (by using the equivalent circuit)
Modeling of boost DC-DC Converter
AC Equivalent Circuit Modeling of boost converter
Average inductor voltage: Average capacitor current
Converter Averaged Equations
Buck-Boost Converter
Average the inductor voltage in this manner:
Average the capacitor current in this manner:
Section (5) Power Electronics 2
