Power converter having a low-loss clamp and method of operation thereof

For use with a power converter having a rectifier that receives current from a secondary side of transformer and delivers the current to an output thereof via an output inductor, a low loss clamp for, and method of, attenuating ringing energy in the secondary side. In one embodiment, the clamp includes: (1) an auxiliary transformer coupled across a component in the secondary side and (2) an auxiliary switch, interposed between the auxiliary transformer and the output, that operates as a function of a voltage of the output to cause the auxiliary transformer to receive at least a portion of the ringing energy and deliver the portion to the output.

TECHNICAL FIELD OF THE INVENTION 
The present invention is directed, in general, to power conversion and, 
more specifically, to a low-loss clamp that attenuates ringing energy in a 
power converter and a method of operation thereof. 
BACKGROUND OF THE INVENTION 
Power converters (for instance, DC/DC power converters) are used to provide 
alternate levels of DC voltage from a primary source of DC voltage. These 
converters customarily use switching devices that convert the DC input 
voltage into an AC voltage to drive a primary winding of a transformer 
thereby allowing the voltages in the secondary side of the transformer to 
be selected to meet the load requirements. The switching devices are 
usually operated at relatively high switching frequencies to allow the use 
of smaller components such as inductors and capacitors within the 
converter. As a result, the parasitic or stray inductances or capacitances 
associated with the components of the converter can be reduced. 
The parasitic elements mentioned above, however, generate high frequency 
oscillations that appear as undesired "ringing" waveforms in the 
converter. The ringing waveforms prompt the use of higher rated and higher 
cost circuit components in order to operate in such an environment. 
Additionally, the deleterious ringing causes the converter to be lossy and 
less efficient. Some of the loss manifests itself as undesirable 
electromagnetic interference (EMI) causing regulatory problems which must 
be addressed. Due to the relatively small resistance values of the 
transformer and inductor elements, the ringing energy may only be lightly 
damped in the converter. 
The spurious ringing necessitates that a rectifier (e.g., diodes) with 
higher reverse voltage ratings be employed in the converter. For example, 
if the ordinary reverse voltage in the converter is 200 volts and the 
added ringing voltage generates 400 volts, the rectifier diode must 
conservatively have a reverse voltage rating of about 600 volts. A 600 
volt diode is more expensive and generally generates a larger conduction 
voltage drop than a 300 volt rated diode (which could be used if the 
voltage ringing did not exist). The increased forward voltage drop induces 
additional losses in the converter thereby effecting the overall 
efficiency. 
Conventional ways of reducing the ringing voltage in the converter include 
a "snubber" circuit placed across each rectifier diode which consists of, 
in one example, a resistor connected in series with a capacitor. The 
snubber acts as a damping device to reduce the ringing amplitude by 
dissipating a portion of the ringing energy. While the snubber circuit 
reduces the reverse voltage across the rectifier diode allowing lower 
rated devices to be used, it also reduces the overall efficiency of the 
converter. More specifically, the snubber capacitor causes more current to 
flow through the rectifier diode when it conducts providing additional 
energy losses in the converter. 
Another technique for reducing the ringing amplitude is to place a 
saturable reactor in series with the rectifier diode. The saturable 
reactor is a nonlinear inductor which adopts a lossy characteristic change 
as the current through it increases to a point where the core material 
saturates. The saturation characteristic effectively damps the ringing 
amplitude by dissipating the ringing energy (and reducing the EMI), but it 
tends to become physically hot and, as a result, is often impractical to 
use in the converter. 
Other damping circuits such as active snubber circuits may also be used in 
a variety of schemes to reduce the ringing amplitude. Examples of active 
snubber circuits are illustrated and described in L. H. Mweene, et al., A 
1 kW, 500 kHz, front-end converter for a distributed power supply system, 
Proc. IEEE Applied Power Electronics Conf., March 1989, pp. 423-432; R. 
Redl, et al., A novel soft-switching full-bridge dc/dc converter: 
analysis, design considerations and experimental results at 1.5 kW, 100 
kHz, IEEE Power Electronics Specialists Conf. Rec., 1990, pp. 162-172; G. 
Hua, et al., An improved zero-voltage-switched PWM converter using a 
saturable inductor, IEEE Power Electronics Specialists Conf. Rec., 1991, 
pp. 189-194; K. Harada, et al., Switched snubber for high frequency 
switching, IEEE Power Electronics Specialists Conf., 1990, pp. 181-188; V. 
Vlatkovic, et al., High-voltage, high-power, ZVS, full-bridge PWM 
converter employing an active snubber, Proc. IEEE Applied Power 
Electronics Conf., March, 1991, pp. 158-163. The aforementioned references 
are incorporated herein by reference. The presently available active 
circuits, however, tend to be complex in nature and generally lack the 
robustness inherent with the use of passive elements. 
Accordingly, what is needed in the art is a robust means to reduce the 
undesirable ringing amplitude in the converter without significantly 
effecting the efficiency of the converter. 
SUMMARY OF THE INVENTION 
To address the above-discussed deficiencies of the prior art, the present 
invention provides, for use with a power converter having a rectifier that 
receives current from a secondary side of transformer and delivers the 
current to an output thereof via an output inductor, a low loss clamp for, 
and method of, attenuating ringing energy in the secondary side. In one 
embodiment, the clamp includes: (1) an auxiliary transformer coupled 
across a component in the secondary side and (2) an auxiliary switch, 
interposed between the auxiliary transformer and the output, that operates 
as a function of a voltage of the output to cause the auxiliary 
transformer to receive at least a portion of the ringing energy and 
deliver the portion to the output. 
The present invention therefore introduces the broad concept of capturing, 
in an auxiliary transformer located across a component (such as the 
rectifier, the transformer or the output inductor), at least a portion of 
ringing (AC) energy that may be present in the secondary side of the 
transformer and directing the captured energy toward or delivering the 
captured energy to the converter's output. Capturing and delivering the 
energy in this manner incurs little power loss. In fact, if the auxiliary 
transformer and auxiliary switch are judiciously chosen, the clamp of the 
present invention may be considered lossless in practical terms. 
In one embodiment of the present invention, the auxiliary switch is a diode 
biased toward the output. Of course, the present invention may make use of 
an active switch, such as a bipolar transistor or field-effect transistor 
(FET), under active control to achieve the desired delivery of ringing 
energy. 
In one embodiment of the present invention, the clamp further includes: (1) 
a second auxiliary transformer coupled across another component in the 
secondary side and (2) a second auxiliary switch, interposed between the 
auxiliary transformer and the output. In one embodiment to be illustrated 
and described, multiple auxiliary transformers and switches are employed 
to capture energy from one or more components. 
In one embodiment of the present invention, the auxiliary switch further 
operates to preclude the current from passing from the output through the 
auxiliary transformer. Thus, the switch may completely block reverse 
current flow through the clamp. 
In one embodiment of the present invention, a primary to secondary turns 
ratio of the auxiliary transformer is preselected such that a clamping 
voltage of the auxiliary transformer is lower than a voltage of the 
ringing energy. In a related embodiment, a primary to secondary turns 
ratio of the auxiliary transformer is preselected such that a clamping 
voltage of the auxiliary transformer is higher than a steady state voltage 
across the component. 
In one embodiment of the present invention, the auxiliary transformer (or 
transformers, as the case may be) is integral with the component in the 
secondary side. In such cases the auxiliary transformer is integral with 
the component (the rectifier, the transformer or the output inductor) from 
which the ringing energy is received. Alternately, the auxiliary 
transformer can be a discrete component. 
The foregoing has outlined, rather broadly, preferred and alternative 
features of the present invention so that those skilled in the art may 
better understand the detailed description of the invention that follows. 
Additional features of the invention will be described hereinafter that 
form the subject of the claims of the invention. Those skilled in the art 
should appreciate that they can readily use the disclosed conception and 
specific embodiment as a basis for designing or modifying other structures 
for carrying out the same purposes of the present invention. Those skilled 
in the art should also realize that such equivalent constructions do not 
depart from the spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION 
Referring initially to FIG. 1, illustrated is a schematic diagram of a 
power converter (e.g., a DC-DC power converter) 100 providing an 
environment for the present invention. The converter 100 is a DC-to-DC 
half-bridge topology including input capacitors C1, C2, switching devices 
Q1, Q2, transformer T1, transformer leakage inductance L1, first, second, 
third and fourth rectifier devices D1, D2, D3, D4 and associated rectifier 
device capacitances CD1, CD2, CD3, CD4, output inductor Lo, output 
capacitor Co and output load resistance R1. The switching devices Q1, Q2 
are alternately driven to conduct with a duty cycle that is controlled 
from regulating circuits (not shown) that sense and maintain an output 
voltage, Vout, at its desired value. The duty cycle depends on the ratio 
of output voltage Vout to an input voltage Vin with larger values of the 
input voltage Vin dictating shorter "on" times and vice versa. As the 
switching devices Q1, Q2 alternately conduct, the transformer T1 drives 
the first and third rectifier devices D1, D3 and the second and fourth 
rectifier devices D2, D4 alternately to conduct thereby transmitting power 
to the output of the converter 100. The output inductor Lo and the output 
capacitor Co are filter elements that provide a constant output voltage 
Vout. 
As previously described, the maximum reverse-bias voltage (MRBV) that the 
rectifier devices D1, D2, D3, D4 encounter is a critical parameter 
affecting both converter 100 cost and efficiency considerations. In an 
idealized case, the MRBV equals the output voltage Vout divided by the 
"duty-ratio" (a parameter less than one and designated by the letter D) of 
the switching devices Q1, Q2. Therefore, MRBV=Vout/D in the idealized 
case. 
Turning now to FIG. 2A, illustrated is a simplified schematic diagram of 
the converter 100 of FIG. 1 showing, in particular, the transformer 
leakage inductance L1 and the first and third rectifier device 
capacitances CD1, CD3. This is an equivalent circuit diagram for the 
period of time when one rectifier pair (e.g., the first and third 
rectifier D1, D3) is turning off, the other rectifier pair (e.g., the 
first and third rectifier D1, D3) is turning on and all rectifiers D1, D2, 
D3, D4 are conducting as the load current Io commutates from one diagonal 
pair to the other. As soon as the current drops to zero in the first and 
third rectifiers D1, D3, the reflected primary transformer voltage drives 
a resonance between the transformer leakage inductance L1 and the parallel 
combination of the rectifier capacitances CD1, CD3. As a result, a ringing 
voltage whose peak value equals 2Vout/D or twice the value of the 
idealized case is produced. 
Turning now to FIG. 2B, illustrated is a voltage waveform 250 present in 
the power converter 100 of FIG. 1. More specifically, the voltage waveform 
250 demonstrates a waveform at node X during a period DT when one 
rectifier pair (e.g., the first and third rectifier D1, D3) is conducting 
and during a period (1-D)T when both rectifier pairs (the first and third 
rectifier D1, D3 and the second and fourth rectifier D2, D4) are 
conducting. A ringing voltage whose peak value equals 2Vout/D is 
demonstrated during the period DT when the first and third rectifiers D1, 
D3 are conducting. 
Turning now to FIG. 3, illustrated is an embodiment of a clamp circuit 300 
constructed according to the principles of the present invention. The 
clamp (e.g., a voltage clamp) 300 includes an auxiliary transformer TC, an 
auxiliary switch (e.g., a diode) DC and is coupled to a low internal 
impedance voltage source VB. One side of the secondary of the auxiliary 
transformer TC is connected to the anode of diode DC and the cathode of 
diode DC is connected to the positive terminal of the voltage source VB. 
The negative voltage terminal of voltage source VB is connected to the 
other side of the secondary of the auxiliary transformer TC. 
To use this clamp, the primary terminals of the auxiliary transformer TC, 
are connected across a section of a circuit (for instance, an output 
inductor of a converter) whose transient voltages are to be limited. Any 
voltage connected directly across the primary of the auxiliary transformer 
TC, should have essentially an AC voltage component only and an 
insignificant DC component, since a substantial DC voltage component will 
cause the auxiliary transformer TC to saturate and become ineffective. A 
blocking capacitor Cb may be placed in series with one side of the primary 
of the auxiliary transformer TC, if necessary. 
The primary to secondary turns ratio Np/Ns of the auxiliary transformer TC, 
should be chosen such that the primary transformer voltage, reflected 
across the secondary when the diode DC conducts (that is, the clamping 
voltage) is lower than the transient or ringing voltage to be attenuated 
but higher than any steady state voltage that normally exists across the 
primary terminals. 
The clamping circuit 300 is passive and therefore more robust than an 
active circuit. It reduces the voltage stress on, for instance, rectifier 
diodes of a converter, and improves overall system efficiency of the 
circuit employing the clamping circuit 300 due to recovery and redirection 
of the ringing energy into the output load. Further, to achieve component 
efficiency, the clamping circuit 300 may be integrated with other 
appropriate magnetic circuit devices, requiring only an external switch to 
achieve clamping operation. 
Turning now to FIG. 4A, illustrated is a schematic diagram of a power 
converter (e.g., a DC-DC converter) 400 constructed according to the 
principles of the present invention. The converter 400 includes a low loss 
clamp having an auxiliary transformer TC and auxiliary switch (e.g., a 
diode) DC. In this embodiment, the primary input terminals of a low loss 
clamp are positioned across an output inductor Lo and the clamp's output 
terminals are placed across the output of the converter 400. As discussed 
earlier, the clamping voltage supplied by the primary of an auxiliary 
transformer TC, is selected to be slightly greater than the steady state 
voltage that exists across the output inductor Lo. By making the clamping 
voltage only slightly greater than the steady state voltage, maximum 
recovery of the spurious ringing energy occurs. This redirected energy is 
applied to the output of converter 400, providing increased overall 
efficiency by recovering energy that would otherwise be lost in 
conventional methods to control voltage ringing as discussed above. 
Turning now to FIG. 4B, illustrated a voltage waveform 470 present in the 
power converter 400 of FIG. 4A. As discussed earlier with respect to FIG. 
2B, the peak unclamped value of the ringing voltage at node X is 2Vout/D 
and the steady state voltage would be Vout/D if there was no ringing 
voltage. The clamp-reduced ringing waveform 470 with respect to the 
converter 400 shows that the peak ringing voltage at node X has been 
reduced to the value dictated by the primary transformer clamping voltage 
VCLAMP. 
At the instant ringing begins, the leakage inductor current is equal to the 
output current Io. This current increases until the ringing voltage at 
node X reaches the value of the clamping voltage VCLAMP where it limits 
and cannot increase farther. At this point the leakage inductor current 
decreases until it again reaches the value of the output current Io, where 
ringing again ensues but at the greatly reduced peak amplitude of 
VCLAMP-Vout/D. The amplitude and energy of this resonance are much smaller 
than the original unclamped ringing and therefore afford a superior 
environment for critical circuit components. 
Limiting the voltage by clamping across the output inductor Lo causes the 
ringing waveform to be attenuated across the rectifier diodes D1, D2, D3, 
D4 as well. Basically, application of the low loss clamp across the output 
inductor Lo causes the "stiffness" or low impedance of the output of the 
converter 400 to be reflected so that this low impedance stiffness 
characteristic is applied across the output inductor Lo. This means that 
the ringing environment of the rectifier diodes D1, D2, D3, D4 is then 
also highly damped since the rectifier diodes D1, D2, D3, D4 is driving 
these two circuit structures and rectifier diodes D1, D2, D3, D4 ringing 
is constrained to be no more severe than the greatly reduced waveform as 
shown in FIG. 4B. 
It is desirable to integrate the auxiliary transformer TC with the output 
inductor Lo. Several design elements should be balanced for overall 
successful operation. However, it may be necessary to isolate the design 
problems by using a separate auxiliary transformer TC. Of course, the 
auxiliary transformer TC may be placed across other components (preferably 
ones causing the ringing energy) in the secondary side of the transformer 
T1. 
Turning now to FIG. 5, illustrated is another embodiment of a power 
converter (e.g., a DC-DC half-bridge converter) 500 constructed according 
to the principles of the present invention. The converter 500 includes a 
low loss clamp having a first and second auxiliary transformer TC1, TC2 
and a first and second auxiliary switch (e.g., a first and second diode) 
DC1, DC2. The low loss clamp is positioned across a secondary winding of a 
transformer T1 and rectifier diodes D1, D2, D3, D4 and is connected across 
a load of the converter 500. The turns ratio of the auxiliary transformers 
TC1, TC2 should be selected for their primary voltages (the reflected 
output or clamp voltage) to be between the steady-state and peak ringing 
transformer voltage. The ringing waveform exhibited by the converter 500 
is analogous to the ringing waveform with respect to the converter 400 of 
FIG. 4 and as illustrated and described with respect to FIG. 4B. 
Exemplary embodiments of the present invention have been illustrated above 
with reference to specific electronic and magnetic components. Those 
skilled in the art are aware, however, that components may be substituted 
(not necessarily with components of the same type) to create desired 
conditions or accomplish desired results. For instance, multiple 
components may be substituted for a single component and vice-versa. 
Similarly, although a magnetic device having a single core and a single 
primary winding has been illustrated, other configurations, such as 
magnetic devices having multiple primary windings or multiple cores, may 
be used to accomplish essentially the same results disclosed by the 
present invention. Additionally, the concepts of the present invention may 
be employed with other circuit topologies. 
For a better understanding of power electronics, power converter 
topologies, such as bridge-type power converter, and clamping circuits, 
see: Principles of Power Electronics, by J. Kassakian, M. Schlecht, and G. 
Verghese, Addison-Wesley Publishing Company (1991), which is incorporated 
herein by reference. 
Although the present invention has been described in detail, those skilled 
in the art should understand that they can make various changes, 
substitutions and alterations herein without departing from the spirit and 
scope of the invention in its broadest form.