Higher VGS Threshold Voltage Benefits in Resonant Applications

In this article, was investigated the impact of a super junction MOSFET gate-source threshold voltage (VGSth) in soft switching applications, using a half-bridge LLC as a test vehicle. A higher VGS(th) impacts many aspects and introduces certain benefits and disadvantages as well.

 

VGSth Role During Turn-OFF

Our first analysis concerns higher VGSth values during the turn-OFF of an SJ MOSFET. As shown in Figure 1, the threshold voltage defines toff, and the drain current must fall to zero when V_GS reaches the threshold, so a higher V_GSth reduces the t_off, which equates to lower switching losses. We performed a simulation in SIMetrix that reproduces a double pulse test to verify this behavior. The same spice model was used and only the V_GSth parameter was changed. Table 1 shows the results.

 

Table 1: Pulse test result

Pulse test results
	dV/dt [V/ns]	dI/dt [A/μs]	toff [ns]	Eoff [μJ]
VGSth 3V	43.60	655	10.11	11.12
VGSth 4V	46.00	765	9.11	9.01

 

The model we used corresponds with the same MDmesh™ technology from STMicroelectronics, with the only differences being the gate to source voltage thresholds. The simulation confirms an improvement in switching performance for the higher 4V threshold with respect to a 3V threshold.

 

Figure 1: V_GSth role during turn OFF

 

Figure 2 shows that the peak power associated with the higher threshold is lower than the other, and also confirms the shorter toff time associated with the higher threshold.

Clearly, V_GSth impacts the turn-OFF mechanism in hard switching applications in a similar way to soft switching, but the presence of turn-ON losses implies a tradeoff between E_on, E_off, and V_GSth.

 

Application Results

To confirm the simulation results, the same MDmesh™ technology was evaluated in a Half bridge LLC (HBLLC) resonant converter using identical DUTs except for different gate threshold levels (V_GSth = 3V / V_GSth = 4V). Table 2 shows the measured V_GSth values. The HBLLC used is the EVALJIG_HBLLC_v4.1 an open loop converter, rated at 600W. The E_off is taken at 10%, 25%, 50%, 75% and 100% of the Pout (see Table 3), in order to evaluate the differences in terms of switching energy, and how this lower turn-off energy is reflected in terms of higher power efficiency.

 

Figure 2: Poff switching waveforms for VGSth = 4V in green and VGSth = 3V in red

 

Figure 3: Turn OFF waveforms, Vth = 3V

 

Table 2: Measured V_GSth value for the four devices under test

Measured VGSth
	Higher Vth		Lower Vth
VGSth (V) @ 250μA	QHS	QLS	QHS	QLS
	4.10	4.12	2.97	3.05

 

 

Figure 3 and Figure 4 show the waveforms taken with an oscilloscope at 10% of the maximum load. The measurement setup used is the following:

 

• Tektronix AFG 3021, signal generator
• Tektronix DPO 7104C, oscilloscope
• TDK-LambdaGEN 600-5.5, power supply
• Agilent N3300A, electronic load
• Yokogawa WT310, power meter
• Tektronix TCP0030, current probe
• Tektronix P5205A, differential voltage probe
• Tektronix P6139B, passive voltage probe
• Flir E30, thermo camera

 

Table 3: MOSFET turn-OFF energy for differing V_GSth values

Turn-OFF energy
Eoff (μJ)
Pout	VGSth 4V	VGSth 3V
10%	6.67	7.12
25%	10.64	11.54
50%	12.93	14.28
75%	13.5	15.3
100%	14.7	16.95

 

 

Starting from the values in Table 3, it is possible to calculate the power associated with the E_off delta for a known switching frequency using the well-known formula shown in Equation 1.

P_SWoff = E_off ∙ f_SW     Eq. (1)

The results in terms of power efficiency are given in Table 4. Delta power represents the difference in terms of P_in for the bridge sec*on (two MOSFETs) of the DUT with V_GSth = 3V minus the P_in for V_GSth = 4V at the same P_out, considering only the increase P_in due to higher off power. While delta efficiency represents the difference in percentage between 4V and 3V.

 

Figure 4: Turn OFF waveforms, V_th = 4V

 

It is clear that a higher gate threshold leads to higher efficiency, especially at light loads. ΔT represents the difference in temperature for each device with the lower threshold; Equation 2 shows the relationship between power and temperature, where R_thj-amb for a TO-220 package is 62.5 °C/W. It is important to highlight that the MOSFETs used in this test are measured without a heatsink.

 

Table 4: Δpower and Δefficiency

Δpower and Δefficiency
Pout	Eff. VGSth 3V (%)	Eff. VGSth 4V (%)	Δpower (W)	Δefficiency (%)	ΔT (°C)
10%	90.96	91.07	0.134	0.111	4.2
25%	92.05	92.19	0.247	0.140	7.7
50%	92.79	92.88	0.327	0.094	10.2
75%	92.55	92.63	0.409	0.078	12.8
100%	92.04	92.11	0.486	0.069	15.2

 

 

\[\Delta T = \frac{( \Delta power\cdot R_{thj-amb})}{2}\] Eq. (2)               

 

Advantages and Disadvantages of a Higher V_GSth

Below is a summary of the benefits and drawbacks associated with a higher V_GSth

 

Advantages:	Disadvantages:
Smaller turn-OFF *me Lower switching losses Higher immunity to false turn-ON due to noise Lower switching losses	Impact on gate driver Higher di/dt

 

The threshold voltage sets the condition when the current starts to flow in the MOSFET; if there are disturbances in the converter or if the ground plane is weak, some current can flow on the gate resistance (through the parasitic gate to drain capacitance C_GD) and may create a voltage between gate and source terminals that could turn on the MOSFET and thus compromise the reliable operation of the entire system. Higher thresholds give higher immunity because of the voltage required across Gate and Source (i.e., the product of R_Gon and noise current) to turn ON the MOSFET higher.

 

Figure 5: Transfer characteristics for MDmesh™ technology, in purple with VGSth = 3V and in dot blue with VGSth = 4V

 

The higher threshold also mitigates the gate oscillation that occurs during turn-ONand turn-OFF, which can improve the reliability of the device due to lower gate oxide stress.

A major drawback is an impact on the gate driver. In some lighting applications or chargers, the driving voltage is less than 10V; around 6 to 8 V. Figure 5, which shows the transfer characteristics of two devices with respective V_GSth = 3V and 4V values, demonstrates that a higher threshold limits the current capability when the driver is unable to operate with 10V.

Considering all the advantages and disadvantages of higher V_GSth in resonant applications, it seems that this approach is the right option as it improves electrical and thermal performance, especially at light loads, and increases noise immunity as well as mitigating gate oscillation.

 

About the Authors

Domenico Nardo received his Bachelor's Degree in the field of Industrial Engineering and a Master's Degree in the Field of Electrical and Electronics Engineering at the University of Catania. He works as an application development engineer at STMicroelectronics.

Alfio Scuto went to the University of Catania and works as a senior high power RF application engineer at STMicroelectronics.

Simone Buonomo studied at the University of Catania and works as a market and application development manager at STMicroelectronics.