ST AN1316 APPLICATION NOTE

SUBSTRATE

DRAIN

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

BACK METAL

The process has a vertical p-stripe structure, made w ith an array of se ctions, that permi ts an increase o f 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 regio n. Due to this a P-I-N diode is form ed that accounts for t he

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 func tion of tempe rature. A s c an be seen in figure 2, the thermal coe fficient is just 1.7 at 125ºC as oppos ed to greater than 2 in conv entional 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.

January 2001

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AN1316 - APPLICATION NO TE

2. APPLICATION FIELDS.

The MDmesh™ used in the following applic ation can be used in medium power S MPS application s such as those encountered in servers and high-end desktops. Other areas such as portable welding equipment can also benefit from the feat ures of the new device. Advan tages brought to the end user are maximized when al l system implications a re seen and not just the device itself. Switching losses are reduced because of lower intrinsic capacitance, shorter crossover t ime and much smaller gate charge (one-third of that of the conventional devices of similar on-resistance), whereas on-losses are decreased essentially bec ause of R

DS(on)

Figure 2: MDmesh™ R

versus Temperature

DS(on)

2.5 2

1.5 1

0.5 0

MDmesh™ Conventional MOSFET

-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 bet ter 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 marke d 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 V

, Vds, Id, switching characteristics, operating frequency, duty-cycle, switching

energy and steady state heatsink temperature. Based on t he 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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Figure 3: Circuit Configurat i on

AN1316 - APPLICATION NOTE

Vcc

Gnd

LOAD

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

dss

[V]

DS(on)

[Ohm]

@ 25ºC

iss

[pF]

oss

[pF]

rss

[pF]

Package

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 loa ded with 0.084Ohm and the +12V output with 3Ohm, so that the total output power was 348W, very close to the max imum nominal o utput power. The normal operation of Q1 of both MOSF ETs is shown in figure 2 when loaded as mentioned above. The duty-cycle and frequency were not constant, as thought, but variabl e due to the voltage ripple from

the PFC section. The duty-cycle ranged between 37% and 43% and the freq uency betw een 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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