By JC Reynaud F5DKJ

Translation by Georges Mathgen LX1BB

Introduction

The future SATEDU satellite’s power supply was designed and built by the ADREF13 team and in particular by Max (F11GVK), Pierre (F1BQR) and Jean-Claude (F5DKJ) in relation with a student (Norbert) preparing a Master’s degree in Computer Science at the REMPART technical High school. Our mission had to comply with precise criteria (somewhat outside of the classical concepts of the average electronics guy) which had been defined by the AMSAT team and its project manager Ghislain Ruy, F1HDD. Main characteristics of the satellite:

• It has the shape of a cube with 30cm side length; 5 faces are equipped with solar panels (the 6^th holds the launch vehicle adapter)
• The planned orbit has an altitude of 600km with a period around 96 minutes, an eclipse time of 35 minutes and an illumination time of 61 minutes.
• The available power per panel amounts to 10 Watts. As the satellite will be in rotation around its axis, the maximum power will be around 17 Watts.

• The power supply must be autonomous
• The only exterior control should be the command to switch on the power supply at the separation from the launcher with a time delay of 30 seconds
• The solar panels should remain disconnected until 30 seconds after separation from the launcher
• In case of a launch delay, a safety device should keep the solar panels disconnected for a period of 1 month; that same device is also used if the activation command fails.
• Working temperature: -25 C, +65 C
• Output voltage: +5V, 4A and +9V, 4A
• NiCd type batteries with 8 elements rated at 7 Ah
• Available average electric power per orbit: 3.9W
• Peak power of the payload: 20W
• Measurement system for current, panel and battery voltage on an SPI bus
• Mandatory 80% converter efficiency.

Two different types of solar panels are taken into consideration:

• Silicon type with a cell of 10.778 cm2, one panel containing 2x32 cells, delivering 14 V/ 0.778 A or 11.6 W at the beginning of its life cycle (BOL) and 12 V/0.72 A or 8.6 W at the end of its life cycle (EOL)
• Gallium arsenide type with a cell of 19.5 cm2, one panel containing 4x7 cells delivering 16.6 V/.98 A or 14.4 W at EOL and 12 V/.9 A or 11.6 W at EOL.

Synoptic layout – Description

Solar panel and battery current measurement:

To measure these currents, a MAX 471 device is connected in series between the solar panel outputs or before the battery. It delivers a voltage proportional to the drawn current. This voltage is referenced to the mass.

This component is designed as a differential amplifier and is doubled before the solar panels in order to obtain additional security.

The output value amounts to 1v/A.

Regulator and battery charge management:

The regulation and the charge management are controlled by a MAX 1640EEE device. This component is a constant current charge system.

The charge current and the maximum output voltage are programmed by resistors. As the maximum end of charge voltage of a NiCd element amounts to 1.45v, the maximum voltage in our case amounts to 8x1.45=11.60v.

Converter – regulator for obtaining +5v:

We have chosen an LT 1374-5 STEP-DOWN device which delivers +5v / 4A with almost 85% efficiency.

Converter – regulator for obtaining +9v:

Here, the problem gets more complicated (Thanks Ghislain). Indeed, the battery pack can see its voltage vary from the minimum of 8x1v=8v to the maximum 8x1.45v=11.6v. The first solution was to use a STEP-UP device up to 15v, then a STEP-DOWN device for +9v. With such a setup the global efficiency of the power supply degrades to 60% which is unacceptable for the imposed criteria. Therefore we decided to use a "Buck/Boost" converter that allows to fulfill the criteria with only one inductance and one Mosfet transistor.

The circuit is an LTC1625 with Mosfet transistors (IRF7201). The efficiency amounts to 83%, but is limited in current. Therefore we have doubled the converter on the test board.

The batteries:

We chose NiCd batteries (8 elements delivering 1.2v / 7Ah) which should give a capacity of 67 wh at full charge. In order to minimize the memory effect (Inverse Voltage) if one or two elements fail, the assembly contains Schottky power diodes.

Current and voltage measuring system:

This system is built around a MAX186 analog to digital converter. The circuit has 8 analog inputs.

This sub-assembly only measures raw values and sends them to the SPI bus. The scale conversions will be done by software in the onboard computer (to everyone his own problems, hi!)

Security device for solar panel switching:

The security system is activated when the two lithium batteries get connected. The solar panels are disconnected from the regulator and from the batteries. The connection will either be done by

• the 30s activation
• the 30 days activation
• lithium battery discharge

This device uses a MIC5014 "Mosfet driver" circuit combined with an IRF7201 power Mosfet. The command is done by an AQY210 static relay.

Activation device:

Two functions have been integrated:

• activation with a 30s time delay after separation from the launcher
• activation with a 30 day time delay as a safety device in case of a failure of the first function.

The 30s delay is mandatory to avoid damage to the main payload (those who pay for the launch).

The 30 day delay was chosen to cope with the most unfavorable situation where the launch is aborted before liftoff. In that case we can take the satellite down !!!

30s activation function:

Upon detection of the opening of a contact, this system starts the 30s delay device which in turn activates a static relay to switch on the solar panels. The necessary power is generated by a lithium battery.

In the event that the system does not detect the opening of the contact, the 30 day function takes over.

Remark: As this system functions if no current is present, the static relay inevitably switches on the solar panels at the battery’s death.

30 day activation function:

This system feeds a static relay through a second lithium battery whose lifetime is calculated to last 30 days. After that period, the relay gets its N.O. (Normally Open) function back which switches the solar panels on.

Conclusions:

To design and build this prototype brought us an important know how in areas not familiar to us. A lot of documentary research and many mail exchanges with the project leader (Thanks Ghislain for your time) were needed, but it was worth the effort.

We intend to use this study for future autonomous terrestrial beacons.

Now we will start building the flight board, another challenge, and above all we await the future launch with great confidence.