Power MOSFETs for switching power supply requirements

In recent years, the output voltage of power supplies has become lower and lower, and the output current has become larger and larger (some power supply systems output tens of amperes to hundreds of amperes). Therefore, the use of a switching power supply controller in the power supply design, plus multiple drivers and power MOSFETs to form a multi-phase switching power supply can meet this requirement. If a structure consisting of a multi-phase controller and a power MOSFET is used, the application is very flexible. The switch tube and synchronous rectifier tube can be reasonably selected according to the size of the output current, and good conversion efficiency and low ripple voltage can be obtained.

In order to reduce the loss of the switch tube at high operating frequency, the gate capacitance and on-resistance of the switch tube are required to be small; and the output drain current is required to be large. Many power MOSFET manufacturers have developed various special switching power supply MOSFETs with low Qg, low RDS (on) and large ID. This article introduces the N-channel high switching speed power MOSFET designed for switching power supplies launched by Infineon in July 2007. Its model is BSCO16NO3LSG. Its product summary is: VDS = 30V, RDS (on) = 1.6mΩ, ID = 100A.

Main features and related parameters

The main features and relevant parameters of BSCO16NO3LSG:

1. Can adopt TTL logic voltage control;

2. When the TTL logic voltage is high, its on-resistance RDS(on) is very small. For example, when VGS=5V, RDS(on)=1.7mΩ; when VGS=4.5V, RDS(on)=1.8mΩ; when VGS=4V, RDS(on)=2mΩ; and when ID changes in the range of 0~50A, RDS(on) remains unchanged, as shown in Figure 1;

Figure 1 Typical drain-source on-resistance of BSCO16NO3LSG

The product of gate charge Qg×RDS(on) is small, which is beneficial to reduce losses when working at high frequency and has good switching characteristics;

3. Low thermal resistance RJC, RJC = 1℃/W; and in a 40mm×40mm×1.5mm single-layer copper-clad board (epoxy resin PCB), the copper layer area is 6cm2 (copper layer thickness 70μM), and the thermal resistance of the PCB vertically in still air is RJA = 50℃/W. This shows that under certain working conditions, the PCB area required for cooling and heat dissipation of the power MOSFET is not large, and the PCB area can be reduced;

4. Small size PG-TDSON-8 package (a package with a large heat dissipation pad on the back), its size is: 6mm×5mm×1mm. Its back shape is shown in Figure 2;

Figure 2: The back of the BSCO16NO3LSG package has a large heat sink pad

5. Under the condition of a sufficiently large heat dissipation area, VGS = 4.5V, the temperature TC of the device's heat sink is in the range of 25℃～100℃, and its maximum continuous ID can reach 100A. If the area of ​​its heat dissipation copper layer is 6cm2 (RJA = 50℃/W), VGS = 10V, TA ambient temperature = 25℃, its maximum continuous ID is 32A. If the continuous ID is required to be greater than 32A, the heat dissipation area can be increased or a double-layer copper clad PCB can be used.

The relationship between the drain current ID and the temperature TC of the MOSFET heat sink shows that: when VGS ≥ 10V, even if TC = 120℃, ID = 100A can be guaranteed, and the pulse drain current can reach 400A;

6. The maximum power consumption of this MOSFET is Ptot=125W when TC=25℃. This is related to the heat dissipation conditions. If under the conditions of TA=25℃, RJA=50℃/W (i.e. the heat dissipation area is only 6cm2 copper layer), its maximum allowable power consumption is only 2.5W. If you want to increase the maximum allowable power consumption, you can increase the heat dissipation area or use a double copper layer PCB;

7. The output characteristics of this MOSFET are shown in Figure 3, and the transfer characteristics are shown in Figure 4.

Figure 3 Output characteristics

Figure 4 Transfer characteristics

Meeting the requirements of synchronous rectification

In the synchronous rectification circuit, it is composed of a high-side MOSFET (HS) and a low-side MOSFET (LS), as shown in Figure 5. HS and LS are driven by a driver. In normal operation: when HS is turned on, LS is turned off; when HS is turned off, LS is turned on. In order to prevent the situation where HS is not turned off and LS starts to turn on, there is a dead time when HS is turned off before LS turns on.

Figure 5 Synchronous rectification circuit

Since there are inter-electrode capacitances Cgd and Cgs in the MOSFET, when HS starts to conduct, a current Ic gd flows through Cgd, passes through Rgate+Rdrive and reaches the ground, generating a △VGS, △VGS=（Rgate+Rdrive）×Icgd. If △VGS is greater than the VGS (th) of LS, LS may be turned on, that is, both HS and LS are turned on, which will damage the MOSFET. If the ratio of Cgd and Cgs in LS is Cgd/Cgs≤1, this accident of HS and LS being turned on at the same time can be avoided. BSCO16NO3LSG can achieve the best ratio of Cgd/Cgs≈0.48 in terms of technology, making the synchronous rectification work very safe (the typical value of Qgd given in the data is 10nC, the typical value of Qgs is 21nC, and the ratio is 0.476).

Pinout

The MOSFET adopts the PG-TDSON-8 package with good heat dissipation and small thermal resistance RJC, and its pin arrangement is shown in Figure 6. To ensure good heat dissipation, when designing the printed board, the pads of the four drain parallel pins D are connected to the welding surface of the heat dissipation pad on the back, and the size of the three source parallel pads is as large as possible, which is conducive to heat dissipation. A printed board graphic design is shown in Figure 7.

Figure 6 Pinout

Figure 7 A printed circuit board graphic design

BSCO16NO3LSG has many parameters and corresponding characteristic curves. In the OptiMos series of power MOSFETs, there are various packages of N-MOSFETs, suitable for various switching power supplies.