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Understand synchronous rectification for chargers

23 Oct 2015  | Peter Green, Helen Ding

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Step (2) – Triggering

The second step in the sequence is triggering, which can only happen if the controller is armed and occurs when conduction through the SR MOSFET body diode is first detected. In most controllers triggering occurs when a negative drain voltage below a certain threshold (VTRIG) is detected indicating that current is passing through the body diode. A small delay in the internal comparator, combined with propagation delay to the gate driver output contribute to the overall turn on delay (TDON), during which time conduction occurs through the body diode. In converters using very low loss transformers, it is possible during DCM ringing for the drain voltage to reach negative levels below the triggering threshold. In such cases steps must be taken to avoid false triggering.

Step (3) – Switch off

The final step is switch off and needs to happen at the end of the secondary conduction period. Premature or late switch off leads to efficiency loss, therefore switch off accuracy is an important factor in synchronous rectifier control. After switch off, the controller disarms waiting to be armed again to repeat the sequence.After switch off it is possible for there to be enough residual current diverted through the MOSFET body diode to cause the drain voltage to once again fall below the triggering threshold. It is therefore essential that the controller not be armed at this point to avoid switching the gate drive on again.

Synchronous rectifier controller ICs fall into two broad categories when classified according to the control concept used. The first type, which has been used for more than a decade, directly senses the SR MOSFET drain to source measuring a negative voltage drop during the conduction phase, which is compared with internal thresholds VTRIG and VOFF to determine when to switch to MOSFET gate drive on and off. A direct connection is required for this because the switch off threshold is in the region of 10mV and therefore this input cannot be divided to an even smaller value. However, during the period when the primary side switch is on the drain voltage becomes positive and can reach values of over 100V depending on the line input voltage and turn ratio of the transformer.

As a result, direct drain sensing SR control ICs are based on high voltage technologies. HVIC IC technology allows direct connection to the drain allowing direct sensing of the drain to source voltage, which enables very accurate control of switch on and switch off of the gate drive. SR controllers of this type are not able to support continuous conduction mode (CCM) operation without an external signal to tell the gate drive when to switch off, however low power flyback converters operate using quasi-resonant (QR) control and therefore never enter CCM.

The second type of control uses low voltage IC technology usually incorporating a significant amount of digital circuitry to sense the drain voltage through a resistor divider network. Since this method does not allow for sensing the point of switch off, a predictive control scheme is used to determine it. Amongst the schemes used are; current slope sensing to predict the point of turn off and volt-second balancing that calculates this value from the drain voltage during the primary on time and is able to determine secondary conduction time based on a known output voltage. The latter method can also support CCM operation if required provided some dead-time is incorporated to ensure the SR MOSFET is fully switched off before the start of the next primary switching cycle. If at any point the primary and SR MOSFETs were to be on at the same time a very large shoot-through current would be created that could cause failure of the primary MOSFET.

The schematic in figure 1 shows a direct drain sensing synchronous rectification control IC. This type of controller has a very minimal component count and is available in very small packages such as the 5 pin SOT-23. In this case switch on and switch off are detected by sensing the SR MOSFET drain to source voltage. Since the turn off threshold is very small and significant high frequency ringing is present after switch on, a minimum on time (MOT) is incorporated to prevent premature switch off. This is externally programmable by an external resistor. This type of controller sometimes often also includes a minimum on time protection feature so that under very light load conditions when the conduction period is shorter than the set minimum on time the gate drive is disabled for the following cycle.

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