1. Field of the Invention
The present invention relates to a synchronous rectifier DC/DC converter. More particularly, the present invention relates to a synchronous rectifier DC/DC converter using a controlled-coupling sense winding.
2. Description of the Related Art
Most AC-DC switching-mode power supplies (SMPS) for computers and other digital electronic equipment use either flyback or forward converter topologies. These converters typically use PN junction diodes or Schottky diodes as their output rectifiers. The forward voltage drop Vf of a rectifier diode for power supplies ranges from 0.5V to 1.0V. Typically, the loss due to this forward voltage drop amounts to about 4% to 10% of input power.
Power metal-oxide-semiconductor field-effect transistor (power MOSFET) is a majority carrier device. Recent advancements in MOSFET technology have improved the turn-on resistance Rds(on) of a power MOSFET in a small package to less than 10 mΩ. Therefore, improving SMPS efficiency by using power MOSFET as synchronous rectifier to replace PN junction diodes or Schottky diodes is receiving more and more attention.
In a steady-state, before the primary-side power switch Qp turns on, the output current Iout is flowing through D2 and the output inductor Lo. When Qp turns on, the input voltage Vin is applied across n1 winding of the power transformer Tr1. A voltage, Vn2, is induced across winding n2. The magnitude of Vn2 is determined according to Vn2=Vin*(n2/n1).
The gate of Q1 is connected to Vn2, therefore, its conduction time is synchronized to when Vn2 is positive, which is identical to the conduction time of Qp. On the other hand, the gate of Q2 is connected to the low side of n2 winding. Its conduction time only lasts from T2 to T3, or during the reset time of the transformer. But between T3 and T4, the voltage on n2 winding, Vn2, is essentially zero. MOSFET Q2 is turned off since the gate-to-source voltage Vgs of Q2 is zero. The free-wheeling current can only flow through D2, causing higher conduction loss.
This less than full conduction time of the free-wheeling synchronous rectifier is a major drawback in the self-driven synchronous rectifier scheme. Especially at high input voltage and light load condition, the conduction-time of Qp will be even shorter, and the reset time is shorter proportionally. This will result in a poor utilization of the free-wheeling rectifier Q2.
To remedy the less-than-full conduction time of the self-driven synchronous rectifier scheme, several synchronous rectifier control integrated circuits (ICs) are offered commercially using a predictive turn-off scheme.
As shown in
Toff1(n+1)−Ton1(n+1)=Toffp(n)−Tonp(n)−Tdel1−Tdel2
Similarly, the turn-on of Q2 follows the turn-off of Qp with a slight delay Tdel1. Also, the turn-off of Q2 should precede the turn-on of Qp slightly by an amount of Tdel2. This is also accomplished by a predictive method. In another word, the conduction time of Q2 in a new cycle is derived from the Vn2 waveform of the preceding cycle as shown in
Toff2(n+1)−Ton2(n+1)=Tonp(n+1)−Toffp(n)−Tdel1−Tdel2
The predictive synchronous rectifier control method works effectively for converters operating in fixed switching frequency. Unfortunately there are several situations where the predictive method will fail and result in a fatal shoot-through condition. A shoot-through condition is when the primary power switch Qp turns on before the free-wheeling rectifier Q2 turns off, creating a short circuit condition. One situation where Qp turns on unexpectedly against the predictive scheme is the converter operates in variable switching frequency, such as quasi-resonant converters, or converters operating with spread-spectrum switching frequency. Another situation is the forward converter has a green mode where several switching cycles are skipped in a light load condition.
The shoot-through condition can be seen from
As shown in
To prevent the shoot-through current from damaging the synchronous rectifiers, it is necessary to detect the turn-on timing of Qp in order to turn off Q2 quickly. The use of a separate pulse transformer as shown in
Please refer to
The use of a pulse transformer to transmit the switching timing of Qp is very effective. However, the pulse transformer is another transformer which has to be included in the converter circuit and is also subject to the 4000V isolation requirement for the compliance with international safety standards backed by prestigious organizations such as Underwriters Laboratories (UL), Canadian Standards Association (CSA), and International Electrotechnical Commission (IEC). The main drawback of this pulse transformer approach is its bulky size and additional cost.
Therefore there is a need to provide a reliable and noise-free switching signal of Qp for synchronous rectified converters without the need for a separate and costly pulse transformer.
Accordingly, the present invention is directed to a synchronous rectifier DC/DC converter. This synchronous rectifier DC/DC converter features a simple controlled-coupling sense winding on a power transformer. The sense winding provides a reliable and noise-free switching signal in the synchronous rectifier DC/DC converter to limit the current spike in a shoot-through condition.
According to an embodiment of the present invention, a synchronous rectifier DC/DC converter is provided. The synchronous rectifier DC/DC converter includes a power transformer, a first diode, a first MOSFET, and a first controller. The power transformer includes a core, a primary winding, a secondary winding, and a sense winding. The primary winding is wrapped around the core and receives an input voltage of the synchronous rectifier DC/DC converter. The secondary winding is wrapped around the core and provides the energy of an output current of the synchronous rectifier DC/DC converter. The sense winding is wrapped around the core and provides a sense signal. The first diode is coupled to the secondary winding for rectifying the output current. The first MOSFET is coupled in parallel with the first diode. The first controller is coupled to the sense winding and the first MOSFET for turning on and turning off the first MOSFET according to the sense signal.
In an embodiment of the present invention, the primary winding is between the secondary winding and the sense winding. The winding structure of the power transformer has two variations. In the first variation, the primary winding is wrapped around the secondary winding and the sense winding is wrapped around the primary winding. In the second variation, the primary winding is wrapped around the sense winding and the secondary winding is wrapped around the primary winding.
In another embodiment of the present invention, the sense winding is between the primary winding and the secondary winding. The winding structure of the power transformer has two variations. In the first variation, the sense winding is wrapped around the primary winding and the secondary winding is wrapped around the sense winding. In the second variation, the sense winding is wrapped around the secondary winding and the primary winding is wrapped around the sense winding.
In another embodiment of the present invention, the first controller turns on the first MOSFET in response to a falling edge of the sense signal.
In another embodiment of the present invention, the first controller turns off the first MOSFET at the earlier one of a first moment and a second moment. The first moment is predicted according to a rising edge in a first cycle of the sense signal. The second moment is determined according to a rising edge in a second cycle of the sense signal. The first cycle is previous to the second cycle.
In another embodiment of the present invention, the synchronous rectifier DC/DC converter is a forward converter, including a second diode, a second MOSFET; and a second controller. The second diode is coupled to the secondary winding and the first diode for rectifying the output current. The second MOSFET is coupled in parallel with the second diode. The second controller is coupled to the sense winding and the second MOSFET for turning on and turning off the second MOSFET according to the sense signal.
In another embodiment of the present invention, the second controller turns on the second MOSFET in response to a rising edge in a first cycle of the sense signal. The second controller turns off the second MOSFET at a moment predicted according to a falling edge in a second cycle of the sense signal. The second cycle is previous to the first cycle.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
Please refer to
The diode D1 is coupled to the secondary winding n2 and the diode D2 for rectifying the output current Iout. The MOSFET Q1 is coupled in parallel with the diode D1. The Q1 controller is coupled to the sense winding n3 and the MOSFET Q1 for turning on and turning off the MOSFET Q1 according to the sense signal Vn3. The diode D2 is coupled to the secondary winding n2 for rectifying the output current Iout. The MOSFET Q2 is coupled in parallel with the diode D2. The Q2 controller is coupled to the sense winding n3 and the MOSFET Q2 for turning on and turning off the MOSFET Q2 according to the sense signal Vn3. The other components of the synchronous rectifier DC/DC converter in
As soon as Qp turns on, the sense signal Vn3 is induced via the sense winding
n3. The sense signal Vn3 is a better replacement of the signal Vpt in
As shown in
The predictive timing of the synchronous rectifier Q2 controller 820 is also based solely on the sense signal Vn3. The turn-on of Q2 follows the falling edge of the sense signal Vn3 with a slight predetermined or propagation delay. The Q2 controller 820 turns off Q2 at the earlier one of a first moment and a second moment. The first moment (T7 in
When the switching cycle of Qp remains constant, the Q2 controller 820 turns off Q2 at the predicted first moment. When the switching cycle of Qp changes and Qp turns on before Q2 turns off, the Q2 controller 820 detects the rising edge of the sense signal Vn3 and turns off the MOSFET Q2 in response. By this mechanism, the MOSFET Q2 can be turned off immediately to prevent the damage caused by the shoot-through condition.
Ideally, the sense winding n3 provides a clean and reliable waveform Vn3 for the Q2 controller, even during the shoot-through condition between the time T5 and T7. As long as the turn-on timing of the primary switch Qp is correctly provided by the sense winding n3, the Q2 controller can turn off the MOSFET Q2 at the onset of the shoot-through phenomenon. Therefore a reliable sense signal Vn3 keeps the shoot-through current spike below a safe level.
In order to achieve a reliable and noise-free sense signal Vn3 from the sense winding n3, a good understanding of the power transformer winding structure, the coupling coefficient between windings, and the interaction between the primary-side circuit and the secondary-side circuit during a shoot-through situation is required.
In essence, the key to achieve a reliable and noise-free sense signal Vn3 is to increase the coupling between the sense winding n3 and the primary winding n1; and at the same time, to reduce the coupling between the sense winding n3 and the secondary winding n2. The different results can be seen in the following three variations of the winding structure of the power transformer 800.
In a shoot-through condition, the voltage across the magnetizing inductance Lm, Vm, is about one half of Vin, since Lm is much greater than Lk2. Vn2 is essentially 0. But Vn3, with the time constant of less than 1.0 ns (Lk3/Rsen=2 uH/10 kOhm=0.2 nsec), is an instantaneous replica of the Vm.
An alternative embodiment of
An alternative embodiment of
This controlled-coupling sense winding scheme can be applied to forward converter or its variation topologies such as half-bridge and full-bridge converters. It can also be applied to flyback converters operating in continuous-conduction mode, as shown in
However, with the sense winding, n3, the Q1 controller 1310 can detect the turn-on of Qp at T5 through the sense signal Vn3, and turns off the MOSFET Q1 immediately at T6′. This greatly reduces the current spike as shown in the IQ1 waveform in
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.