ST AN1499 Application note

AN1499

APPLICATION NOTE

DESIGNING A LOW COST POWER BOARD FOR THE

ST92141 MOTOR CONTROL MCU WITHOUT USING IPMs

By Motor Control Competence Center

INTRODUCTION

Power Modules ha ve been i n use for t wenty year s in industr ial m otor drive a pplicati ons. F or
power stage designs, they give the advantages of compactness and good thermal behavior.

Over the last few years a new family of Power Modules, called Intelligent Power Modules
(IPM), have tried to take the integration of motor drive power stages a step further.

These IPMs target lower power and lower cost motor drive system s compared to those tar­geted by standard Power Modules.

However it is an open question whether these IPMs suit high volume and very co st-sensitive
applications, such as the household appliance market.

Figure 1. General System View

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Table of Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 INTELLIGENT POWER MODUL ES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 ADVANTAGES OF IPMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.1 Assembly cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.2 Co mpo nent coun t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.3 Reduction i n time to market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.4 Higher reliabi lity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.5 Product compactness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.6 Package ind uc tance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 DRAWBACKS OF IPMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 Lead fram e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.2 He atsink planarity and stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.3 Embedded gate drive & filter cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.4 Ext ernal bootstrap diodes and temperature pro tection neede d . . . . . . . . . . . . . . . . . 5

1.2.5 Co mpo nent choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 AN ALTERNATIVE SOLUTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 ADVANTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.1 Assem bly & m ounting cons iderations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.2 Reduction i n time to market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.3 Reliability considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.4 Therm al management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.5 Gate drive optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 DRAWBACKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Co mpo nent coun t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 PCB connec tions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 MECHANICAL DATA: ST92141-PLATFO RM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 CONTROL BOARD LAYOUT (ORCAD FILES AVAILABLE) . . . . . . . . . . . . . . . . . . 20

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1 INTELLIGENT POWER MODULES

These products integrate in a single transfer molded package, six IGBTs, six free wheeling di­odes and the interfacing circuits needed to enable direct control from a microcontroller.

In their low cost version they do not include the front-end rectification diodes, nor do they have
a switch or diode for active power factor correction.

External circuits are still needed, such as bootstrap supplies, current sensing and filtering, and
auxiliary supply decoupling.

This applic ation n ote a naly zes the a dvant age s an d d raw back s o f this p ower in tegr ation ap ­proach with regard to the constraints of cost sensitive motor drives.

An alternative solution is proposed that fits better to appliance and large volume applications
in term of optimization and cost.

1.1 ADVANTAGES OF IPMS

Out of all the advantages that are claimed, the major ones seem to be the following:

– Less assembly cost
– Lower component count
– Reduction in time to market
– Higher reliability
– Product compactness
– Low inductance package

These are general claims that need to be confronted with reality.

1.1.1 Assembly cost

Assembling an IPM requires placing it on the PCB, wave soldering and later on fixing the heat­sink with s crew s. Th ese o perat ions ar e inde ed less exp ensive th an ass embli ng six d iscre te
components. You should note how ever that if an active power fac tor corrector is needed, ex­ternal discrete power components are required.

1.1.2 Component count

This is a clear advantage because nine components are replaced by each IPM. However the
need for microcontroller and passive component assembly remains and IPMs do not r em o ve
any major manufacturing step.

1.1.3 Reduction in time to market

Layout of IGBT and MO SFET gate drives requires special expertise. U sing an IPM does not
require all this expertise, but a good understanding of EMI and parasitic inductance effects is
still strongly recommended!

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DESIGNING A LOW COST POWER BOARD F OR ST92141 MOTOR CONTROL MC U ...

Assuming this expertise is available, the estimated saving will be hardly be more than the time
it takes to have a coffee break!

1.1.4 Higher reliability

From a silicon point of view , there are still about 1 5 dice inside the modu le with th eir own
MTBFs rela ted to j unction te mpe rature as well as m ore than 40 wire bo nds. So th e syst em
MTBF may increase because the connection and assembly count decreases. However it may
decrease if the heatsink is not perfectly flat below the whole IPM surface.

1.1.5 Product compactness

When just com paring power switches, IP Ms br ing c omp actnes s. But when ta lking ab out the
whole system, the difference i s negligible. Passive c omponents , heatsink, PCB and c onnec­tors are by far the most bulky parts.

1.1.6 Package inductance

The only circuit area where IPMs reduce the parasitic inductance is located between the gate
drivers and the pow er sw itches. T his is true for the h igh side g ate dri vers but not f or the low
side drivers.

Figure 2 shows that during turn-on, the low side gate drive current loop (B) is not internal to the

module but goes outside. In this case, IPMs do not have a significant advantage over discrete
solutions.

Figure 2. Low side gate drive current loop

Half

Bridge

driver

Vcc

A

B

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1.2 DRAWBACKS OF IPMS

Counterbalancing the advant ages listed above , IPMs have m ajor draw backs that m ake their
use in cost sensitive applications rather questionable.

Let’s review these drawbacks:

1.2.1 Lead frame

Due to manufacturing constraints, an IPM has leads on both sides of the pac kage. This means
that the PCB must be installed parallel to the pack age. So the heatsink must have its contact
base parallel to th e IPM and th e PCB. Whe n you are lo oking for a v ery cheap so lution, this
makes the choice of heatsink difficult.

1.2.2 Heatsink planarity and stiffness

Inside the IPM, power switches are soldered directly on a long and thin lead frame. Then this
lead frame is fully molded for isolation between the heatsink and the active parts. As a result,
these IPMs have a low stiffness and need to be assembled on a good quality heatsink to avoid
internal cracks. As a consequence, cheap heatsinks made of metal sheet are not recom­mended. This may lead to additional cost.

1.2.3 Embedded gate drive & filter cost

One sensitive parameter, in terms of optimizing the motor drive cost, is gate drive impedance.
By adjusting this imped anc e properly, you can find th e ri ght c ompr omis e b etwee n f ilter cos t
and heatsink cost. As IPMs do not give access to the gate drive impedance, you cannot adjust
the dV/dt commutation which may lead to additional filter cost.

1.2.4 External bootstrap diodes and temperature protection needed

Most IPMs available today do not hav e over-temperature protec tion. This requires additional
external circuits.

As bootstrap diodes are not integrated in the module, they need to be added externally. This
leads to additional cost.

1.2.5 Component choice

Needless to say the choice of IPMs today is very scarce and does not match the broad range
of power switch and interface circuits. This is a real drawback when cost is all-important.

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2 AN ALTERNATIVE SOLUTION

The basic idea behind the IPM solution consists of reducing the component count, simplifying
assembly and making board layout easier. However it has been shown that the cost benefit is
not always easy to determine when you calculate at system level.

Another way to split the system consists of grouping all the system SMD components on a
small size FR4 board, called the Control Board, and keeping all the power sw itches and dis­crete components on a mother board called the Power Board.

Figure 5, Figure 6 & Figure 7 show an AC motor drive application using this partitioning. An ex -

ample layout and parts list are given in Appendix 2.
It is worthwhile to review the advantages and drawbacks of this new partitioning.

2.1 ADVANTAGES

2.1.1 Assembly & mounting considerations

Figure 6 shows a contr ol bo ard s chem atic im plem enting a m icrocon troller, i ts pe ripheral c ir-

cuits and three High Voltage Integrated Circuits for interfacing directly to the Power Board
schem atic show n in Figure 7. This microcontroller is dedicated to AC motor control and is
housed in a shrink SO34 package (refer to the parts list in Appendix 2 and ST92141 and
L6386 datasheet on http:\\www.st.com).

The size of this type of control board is about 26mm by 87mm. This makes use of available
FR4 hardware . This board can be plugged into the Power Boar d next t o the discrete power
switches. T he Power Board layout is very easy and simple, even if low cost materials like
CEM1 are used. This makes the size of the Power Board smaller even if single side copper is
used.

The total volume of the Control Board and the power switches is very compact.
Moreover, the discrete Power switches can fit many different heatsink configurations, parallel

or perpendicular, with no planarity and stiffness constraints.
Another ad vantag e co mes f rom the s olderi ng proces s d ifferen ti ation: SM D com pon ents are

soldered using a reflow process, discrete components go through solder waves. This im­proves the production yield.

Finally, if an active power factor is needed, it is easy to add another switch to the power stage.

2.1.2 Reduction in time to market

The physical spli t between co ntrol circuits and power parts make the system easy to layo ut
and quick to debug. T he system power r ange or the input front end can be adapted w ithout af­fecting the Control Board and v ice versa. Any change of microcontroll er package or i ts periph­eral circuits does not interfere with the power stage.

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In other words, the same Control Board can fit different Power Boards.
This makes the first design shorter and the future re-design even faster.

2.1.3 Reliability considerations

The reflo w proces s u s ed to sold er the Co nt rol B oa rd is prov en to be m ore rel iable t han the
wave process.

The connection between both boards is done during the wave process. Figure 5 shows a typ­ical implementation and Appendix 1 gi ves the results of the v ibration test performed on this as­sembly.

2.1.4 Thermal management

Assembling the d iscr ete comp onen ts b y c lips e nab les the d issip ation to b e spre ad ov er t he
whole heatsink surface. This avoids concentrating the losses on a small ar ea and allows you
to use cheap heatsink technology made of metal sheet.

2.1.5 Gate drive optimization

Figure 8 shows the influence of the gate drive impedance on the conducted noise. As the

whole gate drive is available on the Control Board, it is easy to adapt the noise level according
to the filter attenuation at any time. This noise level optimization can save time and cost.

Moreover the Control Board design allows t he use of ad vanced High Voltage Integrated Cir­cuits that integrate a bootstrap diode and comparators (refer to L6386 on http:\\www.st.com).

The is true for power switch selection (see the fully insulated TO220 products like
STGP7NB60HDFP on http:\\www.st.com).

2.2 DRAWBACKS

The main drawbacks relate to:

2.2.1 Component count

Compared to the IPM solution, both the Control Board and the Power Board each implement
about six components more.

2.2.2 PCB connections

The double sided Control Board soldering totals 68 contacts that are processed during wave
soldering.

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