Input Power Management

A project log for 350 Watt True Sine Inverter

Functional design revision of the prototype TS350.

Brian CornellBrian Cornell 05/25/2019 at 17:250 Comments

This covers the input power connections, control & protections, and inverter auxiliary supplies.

In keeping with the design philosophy that minimizes the unit’s dependency on the enclosure, 45A Anderson PowerPoles are used to connect to a power source.  They are quite effective in providing low connection impedance and ease of use.  I do have concern for the long-term mechanical durability of the PCB mounts, particularly with a 30mil board, since the insertion force is considerable.  This is why a strap is placed around the connector.  Note that Anderson offers mounting “wings” for this application but they triple the width of the connector footprint.

The entire unit is fuse protected with two paralleled 30A automotive-style MINI fuses.  These are the highest amperage value available in that form factor and overall occupy the smallest space.  The fuses connect to the input of the Solid State Relay (SSR) and power switch S201.  The switch is a low current (rated 5A) that powers the start circuit and auxiliary supply.  Switching of power to the inverter bridge is handled by the SSR.  This scheme avoids the use of a bulky high-current mechanical switch, enables inrush management, and eliminates sparks as a potential ignition source.

The start (switched) circuit is protected by a PCB mounted 1.6A fuse (F203).  It is intended to disable the unit if an internal short develops, or connected with reverse polarity or to excessive voltage.  As described in the Overview of Operation, the start circuit’s main function is to provide over-volt protection and that detail won’t be repeated here.  Essentially it consists of pass transistor Q205 and provides two levels of over-volt protection.  The first level is non-destructive and disables the unit if connected to a source in excess of 16V but less than 30V.  The second layer engages a zener shunt that opens F203.

U201 is a 6W SMPS in a DIP package.  It provides Vcc and, via LDO U202, Vdd supplies to the inverter section.  It is connected to the output of pass transistor Q205.  The auxiliary supply also powers the cooling fans which are switched by Q207 via controller U101.

The SSR consists of two paralleled MOSFETs, Q202 & Q204, that connect the inverter bridge to the power source.  During operation their combined Rds(on) at operating temperatures ~ 90C is less than 4mΩ.  When the unit is first powered on they are off and the bridge capacitors, C301-4, are charged via R210.  D204 prevents power from the inverter bridge from powering the auxiliary supply when the unit is switched off.

The SSR is connected to the positive input and does not provide reverse polarity protection.  The prototype had this but was eliminated to conserve space and reduce cost.  The SSR requires a floating gate drive which is accomplished with a simple flyback circuit.  It is driven directly by controller U101 with a fixed duty cycle, 100kHz PWM.  It provides a stable 15V drive with ~ 0.5V of ripple and a turn-on time less than 100uS.

The inverter bridge contains substantial bulk capacitance - about 4800uF.  This is necessary since the input impedance is unknown:  connection lead length, wire gauge, and impedance of the source.  More capacitance would be better but cost & space become an issue.  The design criteria expects lead lengths of about a meter using 10AWG wire connected to a 12V automotive style power system.  Under these conditions this bulk capacitance can source full current (~48A) for a switching cycle and will keep the switching frequency ripple under 200mV at full load.  Line frequency (60Hz) ripple will be about one volt and is unavoidable.

Ignoring the impedance (e.g. cut-off frequency) of the connection wiring and given:

            (A * s) / V = C                (1)

Rearranging:    (C * V) / s = A                (2)

Solving:        (0.0048 * 0.2) / 0.00001 = 96A    (3)

But extending the time to 4mS, half the width of a 60Hz sine cycle and allowing a 1V drop:

            0.0048 / 0.004 = 1.2A            (4)

This emphasizes the importance of a low impedance source connection.