MM3878 Series
Three Features of the Secondary Side Synchronous Rectification Control IC
MinebeaMitsumi’s MM3878 Series is a series of secondary side synchronous rectification control ICs for AC/DC power supply that achieves high efficiency and safety through unique control. It successfully achieved very high efficiency by replacing a secondary rectifier diode with a synchronous rectification system using MM3878 and a power MOSFET to reduce power loss.
Features 01High efficiency
It dynamically changes turn-off timing according to the state of power supply, reducing power loss and maximizing the effect of synchronous rectification.
Features 02
Highly safe control
It has a high speed turn-off circuit to prevent reverse current in the case of sudden change of load or other abnormalities.
Features 03
Stable operation
The IC's power is supplied from the drain terminal when it is impossible to apply sufficient voltage to the gate.
Comparison of methods for improving power supply efficiency
MM3878 Series Synchronous rectification
Soft switching
Core loss improvement
Light load mode
Improvement method
Improving loss by replacing a secondary rectifier diode with a synchronous rectification system.
Improving loss by changing the switching of a power switch (MOSFET) used on the primary side from the square wave hard switching to soft switching.
Improving core loss by improving magnetic materials in transformers and inductors.
Standby power consumption using a control IC to lower switching frequency at light load.
Feasibility
Effect of improving efficiency
Low cost
The secondary synchronous rectification control ICs not only improve power loss significantly with high efficiency but can also be installed at a lower cost than other methods. This is why they are used in various power supplies.
The IC dynamically changes turn-off timing according to the state of power supply, maximizing the effect of synchronous rectification.
It prevents reverse current in the case of sudden change of load or other abnormalities with high speed turn-off.
MM3878 Series
TON is controlled as drain voltage detects conduction periods on primary and secondary sides.
The IC can operate with low ON resistance of the MOSFET as gate voltage remains high right up until turn-off.
Other companies' ICs
VG voltage is controlled according to the drain voltage level.
The MOSFET's ON resistance is larger due to lower gate voltage, which lowers efficiency. There is a high risk of large reverse current and surge as the IC turns off after confirming the sharp increase of VD.
FeatureAutomatic Minimum On-Time setting
High efficiency/ Ease of design
Point
Proper Minimum On Time is set after load conditions are estimated based on the primary side conduction period (VDH) and secondary side conduction period (VDL).
It is automatically adjusted and requires no design or external parts.
Image (diagram)
FeatureIC's power supplied from the drain (VD)
Smaller circuit board/ Smaller number of components
Point
When the power supply with low output voltage cannot apply sufficient voltage to the gate, the IC can be operated with power supply from a MOSFET drain using a high pressure resistance regulator embedded in the IC.
Auxiliary windings are unnecessary.
The IC can switch the power supply method properly according to output voltage as power supply is turned on/off according to VCC voltage.
Example of a high-side rectifier power supply circuit
Auxiliary windings are unnecessary.
This is the example of high-side rectifier power supply configuration. Auxiliary windings are not necessary even at high-side rectification, and the IC is operated in synchronous rectification mode by supplying power from the drain.
Block diagram in the IC
A high pressure resistance regulator is embedded.
This is a block diagram of a high-pressure resistance regulator embedded in the IC. The IC detects VCC voltage and starts the power supply from the drain when VCC is 7.8 V or smaller and stops the power supply from the drain when VCC is 8 V or larger. This function enables the IC to maintain VCC voltage.
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