Download 06 Control Methods

Table of Contents
6.0 Control Methods
6.1 Voltage Mode
6.2 Current Mode
6.3 Gated Oscillator Control
6.4 Bipolar Transistor Drive
6.5 MOSFET Drive
Assignment Questions
For Further Research
An Introduction to Communications Systems
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6.0 Control Methods
Objectives
The purpose:
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The illustrations for this section come from Switchmode Power Supply
Reference Manual rev 2 by ON Semiconductor.
Most SMPS circuits use a form of PWM (pulse width modulation) or PFM
(pulse frequency modulation) within a feedback loop to control the output
voltage.
6.1
Voltage Mode Control
PWM (Pulse Width Modulation)
The output voltage is compared to a reference voltage. This in turn is compared
to a ramp voltage. This effectively creates a pulse width modulator.
If the output voltage has dropped below the reference level, the ramp signal turns
ON the transistor switch. Since the turn ON point can occur anytime during the
cycle, the ramp is effectively turned into a variable width pulse.
An Introduction to Power Supplies
1-1
Switch Mode Power Supplies
_____ Notes _____
PFM (Pulse Frequency Modulation)
In this system, either the ON time or the OFF time is fixed while the other is
varied. The circuit can be disabled under fault conditions by preventing the out
put from the pulse generator (one-shot) from enabling the transistor switch.
6.2
Current Mode Control
In this circuit, the output voltage and switch (or inductor) current is sensed and
used to modulate the transistor switch. The oscillator fixes the turn ON instant,
but the turn OFF instant is controlled by the voltage and current feedback. This
technique has a very fast transient response time.
1-2
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
6.3
Gated Oscillator Control
This circuit has two control mechanisms: PWM and PFM. The oscillator
determines the ON instant while the feedback loop and reference determine the
turn OFF instant thus creating PWM. However, the oscillator frequency can also
change depending on the current flowing in the circuit thus creating PFM.
The 78S40 chip used in the labs is of this type.
6.4
Bipolar Transistor Drive
Bipolar transistors are current activated devices and can be driven in two modes:
fixed and proportional base drive.
In the fixed base drive mode, the transistor is normally biased OFF and provided
with a large base current pulse sufficient to drive the transistor into saturation.
This has the advantage of being simple but has the disadvantage of not being
particularly fast because of the storage time associated with the transistor
junction capacitance.
In a proportional drive circuit, the transistor is never allowed to saturate, thus
minimizing the stored junction capacitance charge and increasing the switching
speed. Because of the increased circuit complexity, this technique is generally
reserved for high power switching supplies.
An Introduction to Power Supplies
1-3
Switch Mode Power Supplies
_____ Notes _____
1-4
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
An Introduction to Power Supplies
1-5
Switch Mode Power Supplies
_____ Notes _____
6.5
MOSFET Drive
MOS transistors are voltage controlled devices and can be 5 – 10 times faster
than a bipolar transistor. However, the gate of a MOSFET has a relatively high
capacitance, therefore the drive circuit must be capable of sourcing (and sinking)
high surge currents. Bipolar totem pole drives are often used.
1-6
An Introduction to Power Supplies
Switch Mode Power Supplies
_____ Notes _____
Assignment Questions
On-Line Test
Quick Quiz
1.
2.
Composition Questions
To answer these questions, it will be necessary to do some research.
1.
2.
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4.
An Introduction to Power Supplies
1-7
Switch Mode Power Supplies
_____ Notes _____
For Further Research
1-8
An Introduction to Power Supplies