Regulated Power Supplies
Linear Regulated Power Supplies
Switching Regulated Power Supplies
Principle of Operation
Advantages and Disadvantages
Service Precautions
Need for a Voltage Regulators
+
Vi
-
+
Rg
RL
Amplitude
Light load
Amplitude
Increasing line voltage
Vo
-
Vi = Filtered input
Vo = Regulated output
RL = Effective load res.
Rg = Effective
regulator res.
Heavy load
Regulated output voltage
Time
Decreasing line voltage
Time
What is the device who will have a voltage across that decreases
while the current through it increases or vice a versa?
Circuit symbols and i-v characteristics
of a zener diode
The zener diode voltage stabilizer
A Series Regulated Linear Power Supply
Series Regulator with a Current Limiting Transistor
Simplified diagram with IC regulator
Commonly available linear
regulator types
• Non-electrolytic capacitors
across the input and output
terminals
• Selection criteria
– Required output voltage and
current
– Minimum and maximum
voltage drops across the device
– Short circuit protection
A crowbar for over current protection
Zener Diode
(Voltage level
sensor)
+5V
1N52328
5V6, 5%
68Ω 0.1µF
Thyristor (SCR)
Electronic switch
2N4441
The Switching Regulator
iL
Imax
P1
Imin
P2
ton
t
S1
IL
C1
RL
tT
t=0
S1
Pos1
VIN
L1
ton
Pos2
tT
t
Transistor switch S1 chops the input at high
frequencies (f > 20 kHz)
LC integrator smoothes the input to produce a
DC output
L and C store energy during the “on” time and
deliver it to the load in a regulated manner
General Layout of a Switching
Regulator
SENSE SIGNAL
INPUT
RECTIFIER
INPUT
FILTER
RF
CIRCUIT
OUTPUT
RECTIFIER
OUTPUT
FILTER
AC
Input
INPUT CIRCUIT
HIGH
FREQUENCY
TRANSFORMER
OUTPUT CIRCUIT
DC
The Input Rectifier
180 V PK
127 VRMS
Pulsating DC
AC Input
Signal
127 VRMS
π/2
π
Rectifier
diodes
For rectifier diodes:
Peak Inverse Voltage (PIV): 2*VPK + 0.5*VPK
Forward Current (If): 1.5*ILOAD(max) (referred to input)
2π
π
Output Rectifiers
Alternating
signal
Pulsating
DC output
Chopped
DC input
Rectifier diodes
Step-down
transformer
Filters
Amplitude
Light load
IL
Heavy load
Filter
capacitor
Filtered DC
Rectifier diodes
Capacitors for the input 1000 to 2200µF
(sometimes up to 5000µF)
For output filter > 470µF
Working DC voltage rating (WVDC)
Time
The switching network
HIGH-FREQUENCY
TRANSFORMER
SWITCHING
COMPONENTS
UNREGULATED
STEP
DOWN
DC
OUTPUT
RECTIFIER
AND FILTER
NETWORK
REGULATED
DC
OUTPU
MODULATED
SWITCHING PULSES
ERROR
AMPLIFIER
OUTPUT
VOLTAGE
CONTROL
PULSE-WIDTH
MODULATING
CIRCUIT
REFERENCE
VOLTAGE
The buck (step-down) converter
IPK
iL
PWM
CONTROLLER
VC
t
S! CLOSED
OPEN
Tc
t1
0
Inductor current waveform
ic
IPK - ILOAD
ILOAD
∆Q+
0
∆Q-
S1
V IN
t
L1
CR1
iL
C1
Vout
RL
Tc
t1
0
Capacitor current waveform
I = (Vin - Vout)*t/L1 yielding Ipk = (Vin - Vout)*ton/L1.
The duty cycle D= ton/T = ton/(ton+toff) and Vout = VinD
Iin= (Iout*Vout)/(η*Vin) where η is the efficiency of the regulator.
Vripple(min) = IPK*(ESR) where ESR is the series equivalent
resistance of the filter capacitor. (Ipk = (Vin - Vout)*ton/L1)
Advantages and Disadvantages
• Advantages
– Efficiency up to 85% (as opposed to 50% with linear)
– Smaller size
– Much lower power dissipation than linear ones
• Disadvantages
–
–
–
–
Larger ripples
Lower regulation
Do not work under no load conditions
Slow response to transient changes in output (load).
RF interference
• Most of the advantages stated are due to the presence of
the switching transistor. However, in order to achieve that
advantage, the input DC (unregulated) is chopped at a
frequency above 20 kHz.
• Some current designs operate close to 500 kHz and in
near future, up to 1 MHz will be available. Hence, the
operating frequency falls within the RF (radio-frequency)
spectrum.
• As a result each conductor in the high-frequency portion
of the supply behaves as an antenna that transmits those
frequencies to rather long distances.
• This causes interference to power supplies own circuitry,
neighboring sensitive electronic instruments and circuits.
To eliminate RF noise
• Careful grounding and shielding of switching
components and outer case.
• Using well shielded interconnecting cables with
the shield being the common ground to the supply
circuit.
• Using electronic filtering components, such as
capacitors and inductors in the design to suppress
the RF emission.
• Changing physical orientation and position of
components in the supply, as well as location of
the supply itself.
System Dynamics
• Compared to its linear counterparts, the ability of a
switching supply to adjust the output voltage
continually under varying loading conditions is not as
good.
• It is essential to have a minimum load to operate
and it does not work under no load conditions.
• It is also slow in responding to transient changes at
the output (load).
SUPPLY SERVICE PRECAUTIONS
• Be careful of high voltage
– Use extreme caution in taking measurements
– Always unplug supply and allow sufficient time for
large electrolytic capacitors to discharge
– It is also good practice to discharge them manually.
• Watch out for shielding
– Replace and re-solder any shielding and
– Re-secure all grounds before operating the serviced
supply.
• Replacement parts
Replacement Parts
• Use only exact replacement parts. Otherwise, the
switching frequency may shift causing an increased RF
interference.
• Use the same type of components.
• For example if you should replace a tantalum capacitor,
replace it with tantalum of the same value, not with an
aluminum electrolytic capacitor.
• Be careful with calibration
• Unless proper tools and instruments are available do not
attempt to play with calibration adjustments.
• An improper adjustment may degrade the supply just as
much as the use of an improper component.