Patent Application
20080074093
The foregoing features and other features of the present invention will now be described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiment is intended to illustrate, but not to limit the invention. The drawings include the following Figures:
The following definitions are provided as they are typically (but not exclusively) used in the power electronics switching circuit environment, implementing the various adaptive aspects of the present invention.
A body diode is a byproduct of the fabrication process of a semiconductor device, for example a MOSFET. It is an intrinsic “built-in” anti-parallel diode between the source and drain terminals of the MOSFET device.
A control switch is a switch in a switching converter that controls the amount of energy supplied by the source. In a buck converter, the control switch is coupled serially between the input and the load.
A synchronous rectifier switch is a switch in a switching converter that emulates diode behavior. In a buck converter, a synchronous rectifier is configured in parallel with the load.
Cross-conduction is a circuit condition that occurs when both the control switch and the synchronous rectifier switch are “on” simultaneously, effectively shorting the power supply.
Dead-time is a short delay between opening the synchronous rectifier switch and closing the opposing control switch, or between opening the control switch and closing the synchronous rectifier switch, to ensure that both switches are not conducting simultaneously.
A free-wheeling diode, such as a high-speed, low forward-voltage drop diode, is connected across a switch and provides an alternate conduction path through which the load current may flow during the dead-time.
Diode reverse recovery time is the time required for a diode that is conducting forward current to switch from forward conducting to reverse biased, non-conducting. Recovery is a phenomena associated with diodes. A diode requires a finite amount of time to transit from a status of conduction, to a status of non-conduction to be able to physically block current flow.
A pinch-off switch is a switch placed in series with a synchronous rectifier for the purpose of interrupting current in the synchronous rectifier.
MOSFET parameters used in the specification:
Although the circuit configurations illustrated in
When switch controller 106 turns off switch 102, the current in second inductor 114 transfers into diode 111. After switch controller 106 turns on switch 107, the current in diode 111 transfers into switch 107, which has a lower forward voltage drop than diode 111.
Next, when switch controller 106 turns off switch 107, due to the inductances of third inductor 110 and fourth inductor 113, the current in internal switch 108 initially transfers into parasitic or body diode 109. Because of the relatively high voltage drop across body diode 109, the current starts transferring into diode 111. However, at high operating frequencies and high currents, the dead-time between switch 107 turning off and switch 102 turning on does not last long enough to allow the transfer of all the current out of body diode 109 and into diode 111. When switch controller 106 turns on switch 102, diode 111 turns off allowing the voltage at node 116 to rise to VIn 101 (neglecting voltage drop across switch 102).
If body diode 109 is not fully recovered when VIn 101 is applied across switch 107, it causes a large spike of reverse recovery current through body diode 109. This reverse recovery current is sourced by VIn 101 through switch 102. When body diode 109 eventually does recover, it snaps off rapidly. The large current change through the package inductances of first inductor 104 and third inductor 110 generates a large voltage spike on node 116.
Although the timing for internal switches 105 and 108 may be adjusted to minimize the likelihood of cross-conduction occurring, there may still be an excessive ringing at node 116 when switch 105 is turned on (due to the recovery of body diode 109), creating adverse EMI effects.
The operation of circuit 120 is best understood from
In one operational embodiment, at time t0 (
At time t1, switch controller 126 turns off switch 122, and the current in output inductor 134 flows through diode 132.
At time t2, switch controller 126 turns on switch 137, and after a time delay introduced by delay 141, switch 127 turns on at time t3. Under this condition, the current in diode 131 transfers to switch 137 and switch 127, which have a lower forward voltage drop than diode 131. In one embodiment, switch 127 may have a lower breakdown voltage than switch 122. In one alternative embodiment, circuit 120 does not require a time delay for switch 127 turn-on (time interval t3-t2 can be equal to 0), although the time delay may be present.
At time t4, switch controller 126 turns off switch 137, which pinches off the current in switch 127. The current in output inductor 134 pulls down the voltage at node 136, which forward biases diode 131.
After time delay 141, at time t5, switch 127 is turned off. At the moment when switch 127 turns off as a result of the delay function, the current in internal switch 128 has decayed to zero or nearly to zero, so body diode 129 does not become forward biased.
At time t6, switch controller 126 turns on switch 122, raising the voltage at node 136 to VIn 121. Diode 131 stops conducting. Because body diode 129 has not been conducting current, it does not have stored charge and thus does not generate a reverse recovery current spike. Without a reverse recovery current spike, there is no substantial current in package inductances 124 and 130 to cause a voltage spike on node 136.
The switches described above are not intended to be limited to any specific type of switch, and may be, for example, N channel MOSFETS, P channel MOSFETs or any type of synchronous rectifier switch which has a parasitic diode. The switches may also be, but are not limited to, low drain-source on-state resistance switches.
Although the present invention has been described with reference to specific embodiments, these embodiments are illustrative only and are not limiting. Many other applications and embodiments of the present invention will be apparent in light of this disclosure and the following claims.
