Slua 074
Slua 074
Slua 074
APPLICATION NOTE
A NEW FAMILY OF INTEGRATED CIRCUITS CONTROLS
RESONANT MODE POWER CONVERTERS
Larry Wofford
Unitrode Integrated Circuits Corporation
Southeast Design Center
1005 Slater Road, Suite 206
Morrisville, NC 27560
(919) 94l-6355
ABSTRACT
A new family of integrated circuits is introduced. Devices from arc significant differences in features and performance levels
this family implement the necessary architecture to control a broad between the three groups. However, a common operational
range of resonant mode converters. Key features in the areas of philosophy is shared by all: fixed-pulse-width variable-frequency.
switch timing, fault management, and soft-start technique arc This approach has been applied to zero-current-switched (ZCS),
unique to this family. Individual devices arc customized to handle quasi-resonant mode converters with reported success.
off-line or DC to DC. single-ended or dual-switch, zero-voltage-or-
current-switched configurations. Specific application to three Table 1. List of Resonant Mode Control ICs
different resonant mode converters is mentioned.
LD405
GP605
SURVEY OF EXISTING CONTROL CS3805
INTEGRATED CIRCUITS
Since 1986, interest in resonant mode power conversion has UC3860
exploded in the technical conferences. IC makers have been quick
to respond with offerings of control ICs. Table 1 is a list of chips MC34066
available at the present time. To simplify thinking, the first three CS360
parts listed are essentially the same design as arc the last two. There
Fault
Soft-Ref
NI
INV
E/A Out
Range
Rmin
Cvco
Zero
RC
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APPLICATION NOTE U-122
One Shot
Single ZCS
Single ZVS
(UC1864)
Dual ZCS
(UC1865)
Figure 3. Error Amplifier and Voltage Controlled
Oscillator
Dual ZVS
(UC1861) When the output of the E/A is less than or equal to one diode drop
above ground, the VCO operates at minimum frequency. The E/A
output can go as high as one diode drop below 5V. When at this
Figure 2. Output Drive For Different Converters potential, the VCO frequency is at its maximum.
Table 2 details the options implemented in the 1861, ‘64, and ‘65.
Other options can be built from the same die.
Table 2. Implemented Options in the 1861, ‘64, ‘65.
Usable maximum frequency tops out around l.5MHz. The
Device UVLO Vth Outputs Zero-(?)-Switching Frequency range is the difference in equations 2 and 1.
UC1861 16/10V Dual Voltage
UC1864 8/7V Single Voltage (3)
UC1865 16/10V Dual Current
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APPLICATION NOTE U-122
Since the nominal E/A output swing is approximately 3.6V for (8)
full variation in VCO frequency, the gain of the VCO block is
(9)
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APPLICATION NOTE U-122
Figure 7 shows the valid switch windows at 1.2A and 100V for
nominal component values as well as the four tolerance comers.
Several observations can be made. Firstly, the window for the case
of +20% inductor and -10% capacitor variations has zero tolerance.
Zero
Minimum Maximum
Controlled
Pulse Pulse
Pulse
Figure 8. One Shot Timer.
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APPLICATION NOTE U-122
The switch must turn off at 1.30us. This is because the tank
impedance is exactly the ratio of low line voltage to overload
current for these component values. This is the source of the 1.386
factor in equation 5. Secondly, and the point of the illustration, there
is no possible value of fixed switch time that accommodates
component variation.
In figure 8, details of the one shot timer arc seen. The clock signal
from the VCO sets the latch, blanks the output, and causes the RC
timing pin to be discharged. The timing pin determines the
minimum and maximum times the one shot output will be high.
(11)
Between these two limits, the zero detect comparator will Figure 10. UC1864 Steering Logic
terminate the one shot pulse whenever the Zero pin goes below
0.5V. By sensing the zero crossing of the resonant waveform, the
one shot adapts to different resonant component values and varying
line/load conditions. The switch time will properly track the
resonant tank assuring zero stress switching.
SECONDARY BLOCKS
The secondary blocks on board are UVLO, a 5V bias generator,
and fault management with a precision reference. The purpose of
the 5V generator is to provide a stable bias environment for internal
circuits and up to 10mA of current for external loads. The one shot
timing resistor connects to 5V.
UVLO senses both Vcc and 5V. It doesn’t allow operation of the
chip until both are above preset values. When Vcc is below the
UVLO threshold, the 5V generator is off, the outputs are actively
pulled low, the fault latch is set, and supply current is less than
300uA.
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APPLICATION NOTE U-122
UVLO
Fault
Output(s)
Soft-Ref
Figure 12. Fault Comparator, Soft Start, Restart Delay And Precision Reference
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APPLICATION NOTE U-122
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APPLICATION NOTE U-122
220 to 380V
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APPLICATION NOTE U-122
current when the signal is at a high value. The allowable voltage cxample had bipolar current, but a transformer and diode bridge
range at the zero pin is zero to 9V, and resistive current limiting to conditioned the signal for the chip. In this example, zero switch
less than 1mA is sufficient. voltage needs to be sensed for both Q1 and Q2. This poses no real
The half bridge power mosfets arc transformer driven from the problem for Q2. Q1 is another story. Some form of external
differentially connected output drivers of the 1865. A UC3611 circuitry must be employed to sense Q1 and translate the
schottky diode array has been used to prevent the outputs from information to the ground referenced chip.
being forced too far above Vcc or below ground. An easily implemented high voltage comparator circuit is shown
The E/A non-inverting input is directly connected to the Soft-Rcf in figure 17. The pnp and diode are the only high voltage
pin to take advantage of all three features of the pin. This components used. The circuit dissipates only 300mW. The output
emphasizes the simplicity of application of the 1865 to this of this circuit is applied directly to the zero input of the 1861.
converter.
CONCLUSION
OFF-LINE ZVS HALF-BRIDGE A new family of integrated circuits to control resonant mode
CONVERTER APPLICATION converters has been introduced that provides several improved
features over those previously available. This family has parts that
An off-line ZVS half-bridge converter (ref. 3) is shown in figure are suited not only to zero-current-switching, but also to zero-
16. An 1861 controls this converter in much the same manner as voltage-switching converters. The 1861, 1863. and 1865 are suited
the two previous examples and is not shown here. The error amp to off-line ZVS, DC/DC single ended ZVS, and off-line ZCS
configuration matches the ZVS example while the output stage is systems. Controllers for other specific converters can be built from
configured like the ZCS example. this family. Adaptive control for resonant tank component
This application does, however, present a difficulty in sensing variations as well as varying line and load conditions is inherent in
zero voltage to control the one shot. In the first ZVS example, the the chip due to its zero crossing detect circuitry. A unique one pin
voltage waveform was ground referenced and unipolar. The ZCS approach to soft start, restart delay, and system reference provides
adjustable restart delay to soft start time ratios as well as closed loop
control during soft starts. Relative ease of application to three
previously reported converters was discussed.
REFERENCES
1) Andreycak, UC3860 Resonant Control IC Regulated Off-Line
150W Converter Switching at 1MHz, pp 472-481, HFPC’89
Proceedings
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