Current spikes in a buck power converter due to commutating diode turn-off time are eliminated by using a tapped inductor in the converter with the tap connected to the switching transistor. The commutating diode is not in the usual place, but is instead connected to conduct current from one end of the tapped inductor remote from the load during the interval in which the transistor is not conducting. In the case of a converter having a center-tapped (primary and secondary) transformer between two switching power transistors operated in a push-pull mode and two rectifying diodes in the secondary circuit, current spikes due to transformer saturation are also eliminated by using a tapped inductor in the converter with the tap connected to the rectifying diodes and a diode connected to conduct current from one end of the tapped inductor remote from the load during the interval in which the transistors are not conducting

Mclyman, W. T.

English

NASA Technical Reports Server

nited States Patent 4,245,288 
McLyman [45] Jan. 13, 1981 
[54] 
[76] Inventor: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of William T. McLyman, 
Lakewood, Calif. 
ELIMINATION OF CURRENT SPIKES IN 
BUCK POWER CONVERTERS 
[21] Appl. No.: 956,166 
[22] Filed: Oct. 31, 1978 
[51] Int. ( 3 . 3  .............................................. H02M 1/14 
[52] U.S. Cl. ........................................ 363/40; 363/21; 
363/26; 363/47 
[58] Field of Search ....................... 363/21, 26, 39-40, 
363/47, 75, 82,90-91, 101; 323DIG. 1, 17, 
52-53, 55-56; 307/280, 300, 319 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,076,131 1/1963 Weil ................................... 363/91 X 
3,740,639 6/1973 Easter .................................... 363/21 
3,745,443 7/1973 Weil ................................... 363/47 X 
3,870,943 3/1975 Weischedel et al. .................. 363/26 
4,034,280 7/1977 Cronin et al. .................... 363/124 X 
Primary Examiner-William M. Shoop 
Assistant Examiner-Peter S. Wong 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Paul F. McCaul 
[571 ABSTRACT 
Current spikes in a buck power converter due to com- 
mutating diode turn-off time is eliminated by using a 
tapped inductor in the converter with the tap connected 
to the switching transistor, without the commutating 
diode in the usual place, and instead connected to con- 
duct current from one end of the tapped inductor re- 
mote from the load during the interval in which the 
transistor is not conducting. In the case of a converter 
having a center-tapped (primary and secondary) trans- 
former between two switching power transistors oper- 
ated in a push-pull mode and two rectifying diodes in 
the secondary circuit, current spikes due to transformer 
saturation are also eliminated by using a tapped induc- 
tor in the converter with the tap connected to the recti- 
fying diodes and a diode connected to conduct current 
from one end of the tapped inductor remote from the 
load during the interval in which the transistors are not 
conducting. 
7 Claims, 16 Drawing Figures 
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.S. Patent Jan. 13, 1981 Sheet 1 of 5 4,245,28 8 
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4,245,288 
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FIG. 2 illustrates waveforms of current through the 
switching transistor and commutating diode of the buck 
power converter in FIG. 1. 
FIG. 3 illustrates a prior-art circuit diagram of a pulse 
5 width modulated buck power converter with two 
switching transistors and a center tapped (primary and 
secondary) transformer operated in a push-pull mode 
with two rectifying diodes in the secondary winding 
circuit. 
tance and switching diodes of the circuit of FIG. 3. 
the circuit of FIG. 3. 
ELIMINATION OF CURRENT SPIKES I N  BUCK 
POWER CONVERTERS 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
(72 Stat. 435; 42 USC 2457). 
Aeronautics and Space Act of 1958, Public Law 85-568 10 FIG. 4 illustrates waveforms of current in the induc- 
FIG. 5 illustrates a B-H loop for the transformer in BACKGROUND OF THE INVENTION 
This invention relates to DC-to-DC power convert- FIG. 6 is a waveform of input current to the primary 
ers, and more particularly to a technique for eliminating 15 winding of the transformer of FIG. 3. 
current spikes generated by turn-off of commutating 
diodes in buck power converters, and in the case of a 
buck power converter with an isolating transformer 
between rectifying diodes and a switching transistor, 
for eliminating spikes due to turn-off of the rectifying 20 switching diode CR3. 
diodes and to transformer saturation while both switch- 
ing transistors are turned off. 
FIGS. 74 and 76 illustrates buck power converters 
corresponding to the converters of respective FIGS. 1 
and 3 with the improvement of the present invention 
comprised of tapped inductance L1 and an additional 
FIGS. 8-11 ihstrafe current waveforms through 
switching diodes CR1 and CR2 for different output 
There has been premature failure of switching power inductor winding ratios of a center-tapped (primary and 
transistors in buck converters due to current spikes secondary) transformer in the circuit of FIG. 7b. 
25 FIGS. 12 and 13 illustrate waveforms of collector-to- 
emitter voltage with and without the improvement of 
generated in the converter that have been determined to 
) transformer also 
out the improvement of the pres- 
TION OF PREFERRED 
eeding with a description of preferred 
a problem of the prior-art buck power e. While there may be 
a particular converter 
ource B is conducted by the transistor dur- 
' period. This current, labeled Ipin FIG. 1, 
FIG. 2 as a pulsating current. While the 
is turned off, the diode CR1 is switched on 
current ICR as shown in FIG. 2. Although 
spikes due to diode switching are eliminated by use o f a  45 shown with a 50% duty ratio, it is clear that the period 
tapped inductor for the inductance having its tapping ofthe on Pulses may be increased Or decreased, to regu- 
terminal connected to the switching transistor and the late power to the load aS the transistor iS turned On at 
commutating diode connected to conduct current from Some constant switching frequency. An inductor L and 
one end of the tapped inductor remote from the load capacitor C filter the current pulses applied to the load. 
during the interval in which the transistor is not con- 50 The problem with such a buck converter is premature 
ducting. In such a converter also having an isolating failure of the switching transistor when used in switch- 
ing regulator circuits. The cause of such failures has transformer between two switching power transistors been undue stress arising from large current spikes. 
Others have directed effort to elimination of such spik- operated in a push-pull mode and two rectifying diodes, 
odes and the commutating diode is connected to the one ination or minimization of spiking arising from 
end Of the tapped inductor remote from the load to which are different from those previously recognized 
eliminate current spikes due to a rectifying diode being and treated by others. The of spiking to which 
turned off and due to transformer saturation while both this invention is related are the turn-off time of the 
transistors are turned off. 60 commutating diode CR1, and in the case of using a 
The novel features of the invention are set forth with transformer ~1 with two power transistors Q ,  and Q*, 
particularity in the appended claims. The invention will the turn off-times of rectifying diodes CR1 and CR2 
best be understood from the following description when shown in FIG. 3. 
read in conjunction with the accompanying drawings. Transistor Q1 and rectifying diode CR1 in the con- 
the tapping is connected to the rectifying di- 55 ing by various means. This invention is directed to elim- 
BRIEF DESCRIPTION OF THE DRAWINGS 65 verter of FIG. 3 occupy positions very similar to that of the transistor QI and commutating diode CR1 in the 
FIG. 1 illustrates a prior-art circuit diagram of a basic converter of FIG. 1. For that reason the same reference 
numerals are being retained while a second transistor buck power converter. 
4,245.28 8 
3 
Q2 and a second rectifying diode CR2 is added in the 
circuit of FIG. 3. Current spikes due to the turn off of 
the rectifying diodes CR1 and CR2 are very similar to 
the current spikes due to the turn off of the commutat- 
ing diode CR1 in the circuit of FIG. 1, although the 
rectifying diodes CR1 and CR2 in the circuit of FIG. 3 
also give rise to other current spikes, as will presently be 
described. 
In a basic buck converter circuit shown in FIG. 1, the 
function of the diode is to provide a current path for the 
inductor L during the interval in which the transistor 
Qi is not conducting. The load current “commutates” 
between the transistor and the diode. The relationship 
between transistor current IQ and diode current ICR is 
shown in FIG. 2. The spikes in the current waveforms 
occur when the diode turns off and is due to a phenome- 
non known as “minority-carrier sweepout.” The effect 
is that, immediately after the transistor Q1 is turned on, 
the diode acts as a short circuit for a finite period of 
time, thereby causing a current spike through the tran- 
sistor. The transistor will fail if the current values be- 
come large. 
The same problem of diode turn-off exhibited by the 
basic buck converter circuit of FIG. 1 is also exhibited 
by other switching regulators utilizing the buck con- 
verter. The pulse-width modulated converter shown in 
FIG. 3 is the most common design in switching circuits 
presently available, and it also has diode turn-off prob- 
lems, as noted hereinbefore. The circuit of FIG. 3 com- 
bines a pulse width modulator (PWM) with two transis- 
tors QI and Q2 in a push-pull converter arrangement. 
The transformer T1 in the circuit is another source of 
transistor current spikes which the innovation elimi- 
nates. The current spikes are introduced when the trans- 
former core saturates, as will now be described. 
Assume that the transistors operate at a 50 percent 
duty ratio for the waveforms of FIG. 2 which have 
been idealized. FIG. 4 shows the load current, IL, and 
the switching diode currents I c R l  and ICR2 shown. The 
spiking problems of the power transistors are related to 
two features of the diode waveforms. The diode cur- 
rents are not zero prior to power transistor turn-on, and 
the currents are not equal when both power transistors 
are turned off. 
The effect of the diode currents not being zero prior 
to transistor turn-on is that the diode which is being 
turned off acts as a short circuit causing a large current 
to flow in the power transistor, as described with refer- 
ence to FIGS. 1 and 2 for the basic buck converter. The 
effect of the unequal diode currents is that the trans- 
former core saturates as shown in FIG. 5 which plots 
4 
and the diodes CRB and CR2 in the converter of FIG. 
76, causing them to turn off and causing current to flow 
through the diode CR3. When the diode CR3 is turned 
off, the inrush current is limited by the inductance of the 
5 overwind, which is the turns of the inductor between 
terminals 11 and 12 as shown in Fig. 7a. The spiking due 
to diode turn-off is eliminated with the proper amount 
of overwind. Since the diodes are turned off, a current 
imbalance does not exist in the secondary, and the core 
10 will not saturate in the converter of FIG. 7b. 
The amount of overwind needed to turn off the two 
diodes CRI and CR2, which is dependent on the leak- 
age inductance of the output transformer T1, can be 
determined empirically. FIGS. 8 to 11 show the current 
15 flow through diode CRP (or CR2) for different output 
inductor winding ratios. For the data shown in FIGS. 8 
to 10, the output transformer was a C core which has a 
large leakage inductance (Le. Lp110p.h). A toroidal 
core was used in FIG. 11, and it has a much smaller 
By varying the turns ratio of the two windings of 
inductor LP, control of the time in which the leakage 
current goes to zero can be effected. Thus the higher 
the reverse voltage to diodes CR1 and CR2, the faster 
The voltage across the leakage inductance of the 
20 leakage inductance. 
25 the leakage current falls to zero. 
transformer is 
or 
35 (N2/N) (Vo+ 1)- I=Ll(Ai/At) 
where: 
Vd is the diode voltage drop and it is assumed to be 1 
volt. 
Ai& 
40 N=Ni+N2 
Ll=leakage inductance of the secondary of the trans- 
At=time for current to decay to zero 
former 
When the leakage inductance is small, as in the case of 
45 a toroidal core, the two rectifying diodes can be turned 
off very rapidly. FIGS. 12 and 13 show collector to 
emitter voltage with and without the tapped inductor, 
and FIGS. 14 and 15 show the collector current with 
and without the tapped inductor. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
50 
4,245,288 
5 6 
sistor is off, whereby current spikes due to minority-car- isolating transformer in a push-pull mode, the improve- 
rier sweepout time in the commutating diode is elimi- ment comprising a tap in said inductor between ends 
nated. thereof, and a commutating diode, said tap being con- 
2. A buck converter as defined in claim 1 including a nected to said rectifying diodes, and one end of said 
transformer for coupling said switching power transis- 5 inductor, remote from the other end connected to said 
tor to said inductor with a rectifying diode in series load, being connected by said commutating diode to the 
between the,secondary winding of said transformer and center tap of said secondary winding of said isolating 
said tap of said tapped inductor. transformer, said diode being polarized to conduct 
3. A buck converter as defined in claim 1 including a while said transistors are turned off, whereby current 
second switching power transistor and a transformer 10 spikes due to minority-carrier sweepout time in the 
for coupling said switching power transistors to said commutating diode is eliminated. 
inductor, said transformer having a center tapped pri- 6. In a buck power converter for converting DC 
mary winding with ends thereof connected to said voltage from a source to DC voltage applied to a load, 
switching power transistors and the center tap thereof said converter having a switching transistor and a com- 
connected to a return current path for said source of 15 mutating diode to alternately connect said source of DC 
power and a center tapped secondary winding with voltage and circuit ground to an inductor in series with 
ends thereof connected to said tap of said tapped induc- said load, the improvement comprising a tap in said 
tor through separate rectifying diodes, and means for inductor and a connection from said transistor instead 
alternately turning on said switching power transistors of to one end of said inductor remote from said load 
for a push-pull mode of operation. 20 where said commutating diode is connected, whereby 
4. In apparatus comprising a switching transistor, an current spikes due to minority-carrier sweepout time in 
inductor, a commutating diode and a filter capacitor for the commutating diode is eliminated. 
converting DC voltage from a source to DC voltage 7. In a push-pull buck power converter for convert- 
applied to a load by switching power through said tran- ing DC voltage from a source to DC voltage applied to ’ 
sistor and inductor into said capacitor, said capacitor 25 a load, said converter having an isolating transformer 
being connected in parallel with the load, the improve- with a center tapped primary winding and a center 
ment comprising a tap in said inductor between ends tapped secondary winding, two switching transistors, 
thereof, said tap being connected to said transistor, and one transistor for connecting said source between the 
one end of said inductor, remote from the other end center tap of said primary winding and one end terminal 
connected to said load, being connected by said com- 30 thereof, the other transistor for connecting said source 
mutating diode to the return current path from said load between the center tap of said primary winding and the 
to said source, said commutating diode being polarized other end terminal thereof, and means for alternately 
to conduct while said transistor is turned off, whereby switching said transistor on for pulse width modulated 
current spikes due to minority-carrier sweepout time in intervals, two rectifying diodes connecting the ends of 
the commutating diode is eliminated. 35 said secondary winding to an inductor in series with 
5. In apparatus comprising two switching transistors, said load connected to the center tap of said secondary 
means for alternately switching said transistors on, an winding, the improvement comprising a tap in said 
inductor and filter capacitor, an isolating transformer inductor and a connection from said rectifying diodes to 
having a center tapped primary winding and a center said tap in said inductor instead of to an end thereof 
tapped secondary winding, with rectifying diodes cou- 40 remote from the load, and a cornmutating diode con- 
pling the ends of said secondary winding to said induc- nected between the end of said tapped inductor remote 
tor in series with a load, said load being connected in from said load and the center tap of said secondary 
parallel with said capacitor between said inductor and winding of said isolating transformer, whereby current 
the center tap of said secondary winding, for converting spikes due to minority-carrier sweepout time in the 
DC voltage from a source to DC voltage applied to said 45 commutating diode is eliminated. 
load by switching power through said transistor and . + * I *  
50 
55 
65 


Elimination of current spikes in buck power converters

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