AN-9902
Application Note
Two and four-quadrant DC/DC
converters with SCALE drivers
Notes for the use of dead-time logic
by Heinz Rüedi, CT-Concept Technology Ltd. Switzerland
Introduction
Whereas the most widely used DC/DC converter topologies operate only in singlequadrant mode, a 2 or 4-quadrant control range is required in some applications.
These include DC/DC converters which can transport energy in both forwards and
reverse directions as well as buck/boost converters. Figure 1 illustrates the basic
circuit of a 2-quadrant buck converter.
+Vin
Q2
+Vout
+
C1
+
Q1
GND
GND
Fig. 1 Two-quadrant buck converter
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AN-9902
Application Note
The circuit can be rotated to obtain a 2-quadrant up-converter. Only one switch is
clocked, depending on the desired direction of energy flow. The opposite transistor in
each case has no importance as only the freewheeling diode is used. The two
switches Q1 and Q2 must never conduct simultaneously; provision must be made for
a dead time when changing over between Q1 and Q2.
Use of SCALE drivers
SCALE drivers are also ideal for applications of this kind. Firstly in the normal way
where both transistors are always operated in half-bridge configuration, so that the
SCALE driver (in half-bridge mode) can also generate the required dead times.
RC1
RC2
6
3
Mode Selector
7
V+
Channel 2
VCC 13
VL
GND 12
Pulse
Stop
Reset
Transformer
Channel 2
11
Logic
9
VL 2
Input A 4
Hi
Dual
Mode
Dead-
Logic
Time
Trafo
Interface
Error
Logic
Error
Lo
Reset
Channel 1
Input A
Input B 5
Input B / Enable
Pulse
Transformer
Channel 1
16
Logic
14
Status 2 8
Error Channel 2
Status 1 1
Error Channel 1
CONCEPT LDI 001
Trafo
Interface
Error
Logic
Error
Reset
Fig. 2 Block diagram of the LDI 001 logic-to-driver interface
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AN-9902
Application Note
If only Q1 or Q2 are to be clocked depending on the desired direction of energy
flow, the driver’s direct mode can be used. The controller must then prevent both
transistors from conducting simultaneously and must ensure that a specific dead time is
maintained when changing over between Q1 and Q2 and vice-versa.
But there is also an easier way – by looking more closely at the inside of the circuit.
The input signals are processed in the LDI 001 (an ASIC used in the SCALE driver).
However, the block diagram of this ASIC (Fig. 2) does not reveal any details of how
the signals are actually processed.
Figure 3 shows the signal processing in half-bridge mode. The signal applied to input
A is fed to the dead-time circuits – once inverted and once not. An AND gate is
located behind the input; both outputs can be deactivated with input B.
Dead-
Hi
Channel 2
Time
Input A 4
Dead-
Lo
Channel 1
Time
Input B 5
Input B / Enable
(Circuit inside LDI 001)
Fig. 3 Circuit principle in half-bridge mode
If the information about the direction of energy flow is applied to input A and the
PWM signal to input B, then the required characteristic is obtained with no additional
circuits:
:
:
Either Q1 or Q2 can be used for clocking, the opposite switch is always
blocked. The driver applies the PWM signal present at input B in undistorted form
(without adding a dead time) to the corresponding power switch.
Whenever switchover takes place between Q1 and Q2 and vice-versa, the
driver generates a dead time.
Figure 4 shows the resulting signal profile.
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AN-9902
Application Note
Input A
Input B
Gate G1
Gate G2
Deadtime between Q1 and Q2
Fig. 4
Signal profiles in the application
Supplementary notes
We take great care in preparing our application notes. However, we cannot
guarantee that the circuits and procedures shown will operate without problems in
every application. This must be established by the user himself. Neither can we
guarantee that the suggestions are free of third-party rights.
Manufacturer
Your Distribution Partner
CT-Concept Technology Ltd.
Intelligent Power Electronics
Hauptstrasse 3
CH-2533 Leubringen / Evilard
(Switzerland)
Tel ++41 (0)32 / 322 42 36
Fax ++41 (0)32 / 322 22 51
E-Mail info@ct-concept.com
Internet www.CT-CONCEPT.com
Internet www.IGBT-Driver.com
 Copyright 1997...1999 by CT-Concept Technology Ltd. - Switzerland.
We reserve the right to make any technical modifications without prior notice.
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All rights reserved.
Version from 24.05.99
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