ST AN1316 APPLICATION NOTE

AN1316

APPLICATION NOTE

EVALUATION OF THE NEW HIGH VOLTAGE

MDmesh™ VERSUS STANDARD MOSFETs

F. DiGiovanni - M. Laudani - M. Saggio - R. Scollo

1. ABSTRACT

The Multiple Drain Mesh, better known as MDmesh™, is a revolutionary technology as well as a “conceptual” breakthrough in the high voltage power MOSFET area. It is named after the combination of a new vertical drain structure with STMicroelectronics’ well established Mesh Overlay™ layout.

Figure 1: MDmesh™ Structure

N-SOURCE

P-MESH

GATE

FINGER

DRAIN

SUBSTRATE

BACK METAL

The process has a vertical p-stripe structure, made with an array of sections, that permits an increase of more than two times that of the average voltage breakdown. As a result, it is possible to cut the on-resistance within the range of 3 to 4, depending on the voltage rating. In fact, the new approach substantially reduces the resistance of a conventional lightly doped drain. This vertical structure achieves a very good charge balance in the drift region. Due to this a P-I-N diode is formed that accounts for the device's voltage blocking capability. A MDmesh™ MOSFET designed to withstand 500V now exhibits the same drain resistance and lower thickness than those of a conventional 200V MOSFET with a much lower on-resistance. The new drain structure has been coupled to the STMicroelectronics’ Mesh Overlay™ horizontal layout which has enabled ST designers to maintain a perfect control of the internal gate resistance, in addition to substantially reducing gate charge. Another advantage over standard products stands in the law of the on-resistance variation as a function of temperature. As can be seen in figure 2, the thermal coefficient is just 1.7 at 125ºC as opposed to greater than 2 in conventional high voltage MOSFETs. The final result is reduced power dissipation which makes improved system efficiency whereas the lower gate charge implies using smaller and more economic gate drives.

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AN1316 - APPLICATION NOTE

2. APPLICATION FIELDS.

The MDmesh™ used in the following application can be used in medium power SMPS applications such as those encountered in servers and high-end desktops. Other areas such as portable welding equipment can also benefit from the features of the new device. Advantages brought to the end user are maximized when all system implications are seen and not just the device itself. Switching losses are reduced because of lower intrinsic capacitance, shorter crossover time and much smaller gate charge (one-third of that of the conventional devices of similar on-resistance), whereas on-losses are decreased essentially because of RDS(on).

Figure 2: MDmesh™ R	DS(on) versus Temperature
	3	MDmesh™			Conventional MOSFET
	2.5
	2
	1.5
	1
	0.5
	0
	-50	-25	0	25	50	75	100	125	150
				Temperature [°C]

3. INTRODUCTION.

The purpose of the following analysis is to compare the electrical and thermal performance of the STW15NB50 standard MOSFET (500V, 0.36Ohm max, TO-247) with the new MDmesh™ (500V, 0.4Ohm max, TO-220) in a 360W power supply. The STW15NB50 already shows a better on-resistance than most similar 0.4Ohm industry products with its 0.36Ohm max. So it is expected that any performance gap between the MDmesh™ and similar competitors’ devices can be wider than it is in our case. Also, in this analysis one can see how the new technology opens up new challenging opportunities to power conversion designers.

In the tests, the two MOSFETs were mounted in positions marked Q1 and Q2, as shown in figure 3 and under maximum nominal working operation. Our attention was focused on electrical and thermal parameters such as Vgs, Vds, Id, switching characteristics, operating frequency, duty-cycle, switching energy and steady state heatsink temperature. Based on the measurement results, some conclusions were drawn in order to evaluate all performance improvements and energy saving with the new MDmesh™ technology.

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AN1316 - APPLICATION NOTE

Figure 3: Circuit Configuration

Vcc

	Q1	D
		D2

G S

LOAD

D

Q2

D1

G

S

Gnd

4. STATIC CHARACTERISTICS AND CIRCUIT CONFIGURATION

In the circuit, the devices are connected in an asymmetrical full bridge configuration (figure 3), where the load is the primary winding of a transformer.

The main static characteristics of the two devices under test are summarized in the below table.

Table 1: Main Electrical Characteristics

	Vdss	RDS(on) @ 25ºC	Ciss	Coss	Crss	Package
	[V]	[Ohm]	[pF]	[pF]	[pF]
STW15NB30	>500	0.35	2,600	330	40	TO-247
MDmesh™	>500	0.37	930	160	25	TO-220

5. TEST DESCRIPTION AND OPERATION

The unit was supplied from 220VAC. Then, the +5V output was loaded with 0.084Ohm and the +12V output with 3Ohm, so that the total output power was 348W, very close to the maximum nominal output power. The normal operation of Q1 of both MOSFETs is shown in figure 2 when loaded as mentioned above.

The duty-cycle and frequency were not constant, as thought, but variable due to the voltage ripple from the PFC section. The duty-cycle ranged between 37% and 43% and the frequency between 110kHz and 120khz.

Figures 4 and 5 show the turn-off of both devices. The MDmesh™, due to its lower capacitance, switched faster than the STW15NB50 (see table1).

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